I’m a sophomore living in the same dorm as last year, different roommate. My floor is pretty comfortable walking to/from showers in our underwear and last year my roommate and I often hung out in our underwear. I’m a briefs guy and my roommate wore boxer briefs, and everything was cool. This year my roommate loves to humiliate me because of my “tighty whities.” Aside from the occasional comment or whistle in the hall no one really cares. My roommate always makes jokes and it is embarrassing to me. I started wearing shorts in the room and he even pantsed me. What do!?
Relax. Wear what you want.
Learn to live with a jerk until you can get transferred to another room. See if housing can change your room assignment in your dorm.
I consider him to be bullying you. He sounds like he has personal issues with his own body awareness.
- “Dude, why do you keep staring at my underwear?”
- “We aren’t dating, so it shouldn’t matter so much to you”
- Wear a robe/towel/pants
- Pantsing crosses the line, in my opinion…into sexual harrasment…If it happens again, go to your RA
- Start saying to other people “Why is roomie so interested in my underwear…don’t you think that is weird?”
I like number 1 in @bopper 's post above. “Dude, pantsing me is really not cool and I am very uncomfortable with it. Can you stop worrying about my underpants and instead let me know why you care so much about them?”
Or be daring and give boxers a try. 
Or find some new color other than white.
Or go with suggestion no. 1.