Unless you or a third party requires your son to come home for this activity, I think you should ask your son for gas money.
Your son should tell his roommate that he needs to chip in for gas every week or every other week. Figure out what the gas costs you for a round trip and your son should tell his roommate to pay for half of that.
If your son won’t do it, then YOU tell the roommate that this new rule is in effect starting in 2 weeks. The reason you give your son the opportunity to handle it first is so he can learn another lesson in Adulting 101 rather than having Mommy or Daddy call his roommate’s Mommy or Daddy for him.
How do you know the other parents are ok with the current arrangement? For all you know they think it’s stupid their son comes home every weekend; don’t like the GF and don’t know why their kid isn’t meeting women at college; don’t like the fact that their kid takes so much time away from college activities to be on the road. Maybe they are waiting for you to tell your son “If you need to come home, take the bus. But we aren’t providing taxi service to you OR your roommate” and they are wondering how long that’s going to take to happen.
You never know. You feel like you are providing a taxi service. Maybe they think you are being used by both your kid AND their son and are hoping you figure it out before you spend four years enabling this behavior.
The parents are picking up their son anyway. If the roommate doesn’t catch a ride, it costs them the same for gas anyway. The kid is only going home every weekend because there is a ride.
If the parents don’t like it, they should just tell the kid they can’t drive him anymore because they are stopping to visit grandma or going to dinner or need to have a big family talk. If it is really a problem, the parents could give the son the car a year early.
Oh, his parents are thrilled to have him home. They’re a close family. I guess it bothers me because we bought most of the furniture and supplies, we’re the ones refilling their snacks and fridge, we’re the ones stepping up time after time when there’s a need. Now we’re the taxi service. It’s not a question of being unable to afford it, this family is more affluent than we are! I just wish they’d offer something once in awhile so that it felt more equitable. But I like the positive notes posted here, to see it as being fortunate that we are able to do what we can. This is who they are, they aren’t going to change, so I should just focus on doing what needs to be done to make sure my kid has what he needs. Let it go.
My son’s activity is career-related, more like a job, and he needs to do this, so we’re motivated to help him do it.
I appreciate the input, just allowing me to let off a bit of steam has been very helpful! 
OP, I wonder if you can figure out what’s really bothering you in this arrangement. For some reason, it feels like the friend isn’t necessarily it. Is it that you don’t want to be driving period? Do you resent that your son isn’t communicating with you and that the friend enables that? Are you sorry your son isn’t more involved in school? Is your son not thankful enough for the 12 hours a week you devote to driving? Are the friend’s parents rude to you? Or the friend himself?
What would be different if the friend stopped coming and would that make you happy? Even if you drove the same round trip every week?
I am not sure why, if you are happy to drive every weekend, that another body in the car is an issue, especially if he agrees to your time frame. If you want to drive every other week or just in one direction, I would tell your son to take the bus or catch a ride, suggesting that maybe his friend might occasionally be coming home and they should coordinate. But it seems unfair to resent the friend if you are in truth resenting the drive.
My son is happily immersed in school and doing very well. He’s quite appreciative of everything we do for him. I like his roommate, and his roommate’s family, a lot. I suppose I find the drive, when I do it, exhausting, and you’re right, that’s not the fault of his roommate. I think the thing that’s really bothering me is what I mentioned upthread – it’s just one more thing that we’re doing that his roommate’s parents are not. But this thread, reading what you guys have posted, has made me think of it this way: when the guys need something, they come to us. I should take that as a compliment instead of seeing it as a burden.
I’m going to differ from some other posts and say don’t ask for gas money. You’re doing the trip anyway. Another person in the car is adding very, very little to your fuel mileage. This is your son’s friend and roommate. We do nice things for other people who are close to one’s family. Would it be courteous if the boy’s family offered sometime or gave $ or a gift? Of course but that doesn’t change the fact you planned to do it anyway.
I really hate driving, so I’d definitely be chauffeured by those two boys on the return trips. Maybe a book on tape, a little knitting in the back seat…
@booksenkatz , I love that perspective. You are lucky to get the time with them. and they with you…
Is it the actual driving that tires you out or the time spent on the trip? If it’s the driving, maybe @twoinanddone has the right idea. Could either of the students be doing the driving on the leg of the trip when they’re in the car?
I understand how you feel and you have every right to feel the way you do considering the circumstances. My mother always says that it is not always the people that you do good to that is going to do good for you. This means that even though this friend is not reciprocating, there are going to be other people in your life and your son’s life that are going to do good things for you without you doing good things for them.
Here’s another perspective–and I wonder OP if this might be part of it, as clearly you are very generous and giving.
I avoid participating in car pools --and I have been honest when parents ask, and my child is in agreement with this. My kid and I have very little one-on-one time. We have the best conversations, mostly stories that I wouldn’t otherwise hear about school, and significant laughs on this long drive. I jealously guard that precious time.
Another who agrees that you are picking up your son anyway because of the situation he is in so I would not ask for gas money or ask the other family to pick your son up. Let them drive if you need a break from the drive.
I think what you are doing is kind and generous but you are simply getting burned out from doing that long drive every week and that’s creating an opportunity for the resentment to build.
You just need a break. Be honest and reach out to the roommate or his family and ask if they would mind taking a week because you are getting tired out. They might step up, or if their schedule doesn’t allow driving, it might be an opportunity for them to realize the drive is a sacrifice on your part and at least offer to help with gas money.
I also like the idea of you sitting in the passenger seat or the back seat relaxing.
@holychild 's comments are a lot like my experience. When my kids were young, I had a few mom friends who went above and beyond in helping me out in getting my older kids to school and activities. Now I try to do that for other people. At the same time, I have experienced that feeling of resentment when people can help more and don’t. And honestly that’s not good to let fester for the sake of those relationships or for your own peace. So I think you need to deal with it in a positive way.
I hate to say it, but this is a monster that you created. You have shown your kid kindness, generosity and it is not robbery to do something for others and your son is following suit.
If your kid is like mine, he is generous and takes pride in that fact that he can depend on you and you will always come through.
Son: You need a ride home? Don’t worry, my mom has to come pick me up and you can ride with us since my mom is coming to pick me up anyway.
Son: We’re out of snacks? Oh, I’ll just let my mom know and she will have them ready when we come home next weekend.
I am not trying to marginalize your feelings, but I do think that you are getting worked up and letting the situation fester, when you do have options.
I also think that there is a big lack of communication or miscommunication going on. When you bought most of the furniture and supplies, did you have a conversation with the other family or did you simply purchase what you thought your son wanted or needed ?
Unless your child has a medical condition, that needs to be taken care of and as a result he needs to be at home every weekend, his coming home every weekend is a choice. Whatever activity that he is involved in, he is choosing to remain involved in it and you are gave by in by saying "don’t worry, we’ll come get you every weekend if this is still what you want to do (or we think you should continue with this activity).
You are making the choice to refill the snacks and the fridge. If they are out, this is a conversation that you need to have with your son. If he chooses to feed his roommate and the rest of his friends, then have at it, you will not be restocking. He has to pay to restock or chip in with the friends to restock.
You also don’t know the full give and take of their relationship at school. Perhaps, your son may feel that this is a way of giving back carrying his part of the relationship.
I think if you change up, it may be your son who ends up being more disappointed than the friend.
If you don’t want to drive, simple as previously stated; there is a grown man in your car who is probably on your insurance (your son). Let him drive home and on the leg back to school and you drive when you are by yourself.
As for the roommate, I think saying thank you, offering to pay for gas or giving a small token of his appreciation should be done. If his parents know that you are driving him home, it would be nice if they said “books, we appreciate you bringing little Egbert home every weekend, is there anything we can do?”
If you are going continue to do it, do it because it is in your heart do do so. I don’t think that you should do anything begrudgingly or because you expect something in return.
I love my son’s best friends like second sons and would not ask for money or ride-sharing, knowing full well it was something I had to do anyway.
I do think the drive is burning you out, understandably. I would talk with your husband and see if you want to revise the car deal. If your son has a good first semester at college, Let him have the car for second semester instead of waiting for next fall. I really think you spending 12 hours in the car every weekend is just going to make you feel worse as the year goes on. It’s just too much. Just imagine your life with those 12 hours back every week!
What activity is your child involved in that he has to come home every weekend? Is there an end in sight to this involvement?
Agrees that you are picking up your son no matter what so I would not ask for gas money or for the other family to pitch in with driving for these trips. Maybe the other family can do the driving for other reasons like holidays. It would be nice if the family acknowledged your driving with a thank you of some sorts. If all is going well with the boys and families, I would not rock the boat with something that seems petty. Maybe they will surprise you with a holiday gift to thank you for the rides home. I definitely agree to let the boys do some of the driving. My kids share the driving on a lot of long trips. Enjoy the time with both your son and his friend.
I am with the camp that wrote do not ask for anything as you are driving your son anyway . As far as stocking the frig and snacks, I always told DD to share and do not expect anything in return, but if her roommate bought anything, made sure to offer her share.