And if someone offered him a full scholarship to dive with sharks? Extra benefit.
The 2 parts of the original post that struck me are that he wants to play and he and his girlfriend decided he’d follow her to school. OP, if football weren’t involved would you let your son’s girlfriend have so much say in where he goes to college?
If were you, I’d continue to compile a list with the realization that they may not be together next year. Unless you can pay out-of-pocket for whatever school he wants, compile a list of all available options. It’s easier to turn down acceptances than scramble for affordable options in the spring.
There are actually dive scholarships… 
https://www.owuscholarship.org/scholarships/who-can-apply
"My point is that the OP doesn’t care what you think about the risks in football. "
No problem. If pointing out the risks makes one other parent out there consider the risks too high and ends up saving his/her son from brain injury, that’s a good result. At some point if enough people keep deciding to prioritize their sons’ brains over this sport, things will change. OP can ignore if this doesn’t apply to her.
Just watched the movie Concussion. This girlfriend could be saving your sons life. I suggest you read everything you can on CTE. Best of luck to your son.
I would ask your son how important it is for him to play football. IMO if he is willing to give it up for his GF then it is probably not that important. If that’s the case then I would let it go. At the same time, for the condition for me to pay (support) for his college I would want him to apply to a range of schools and I would get few picks. Of course, I would also share with him on how much you could afford. If by the time he wants to make a choice and he still wants to go to his (ex)girl friend’s school then I would let him go.
I do think we should stop preaching about danger of football to the OP. She is not asking us to opine on that. Oh, for everyone who is talking about danger of football, I hope you are not watching any football on TV. I don’t watch it because it is not that interesting to me.
Let’s suppose he could get a full ride scholarship at his pick of public flagships playing golf instead of football. Everyone would say it’s crazy to drop all those other opportunities just to follow his gf in-state.
My father , a neurosurgeon, told us we were forbidden from football way back in the 60s. He would have loved this girl friend.
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Ok so diving with sharks was a bad example. I don” t see how anyone can answer here without addressing the risks. That said I do know couples who started in high school and have been married for decades. A girlfriend might be more important than an interest like a sport and perhaps an affordable option without football can be found. Applying to several options is a good idea in case things change. I spent 10 months helping my daughter recover from a TBI and dont wish it on anyone. Recovery is never complete.
Odds are that the girlfriend won’t stay with a high school kid once she’s been in college for a while. There’s a sharp mental divide for college freshmen, even if they’re the same age! They feel so much older, more mature, more independent, sophisticated, etc. 
So, it’s absolutely essential to keep his college options (with or without football) open - that you determine two affordable academic safeties, 3-5 match schools, etc.
LOL…the quintessential CC thread. OP asks for insight to a specific issue and many responders focus on tangential variable not central to OP’s question. OP attempts to clarify that the tangential variable is not one that needs addressed/solved for them and tries to steer discussion back to original topic. Responders ignore OP and continue their attempt to demonstrate their authority on the tangential variable and convince OP that they know best. Discussion continues on tangential variable. OP disengages from thread. Discussion veers into even lesser-related territories and takes on a mind of its own.
OP, if you are still around, thank you for starting this thread on brain injuries and diving. I think I speak for everyone here when I wish him the best of luck in his recovery and that he is successful in obtaining that elusive scuba scholarship. Hopefully his GF is willing to stick it out as he gains lucidity and strives for underwater glory!
LOL @pishicaca
Too funny, and true, @pishicaca., although to be fair the OP had already received a lot of advice addressing their original question. Sometimes once posters find there’s not much more to say the thread takes on a life of its own (like here where we’re getting all meta and addressing the thread instead of the question!) 
I understand OPs concern. I think that football aside I would be concerned about my child going to a school just because a current GF was going there. If there were other strong reasons he wished to attend then I would be less concerned. While life long commitments can come from HS relationships the track record of such relationships lasting beyond a year into college is not strong. Additionally, she will be a year in before he even gets there. She will have developed new friends, new opportunities to meet people and gained a new maturity (it can be amazing the changes during freshman year). He will still be a HS student with a different view of his situation and things than she. Both my girls had HS relationships they took to college and neither lasted beyond the summer after their freshman year. There is a likelihood that she will move on before he even gets there. How self confident is he? Do you think he would rebound well from a breakup or would it affect his academic performance especially if she were around?
There are definitely opportunity costs related to football here. It sounds like he has already decided that it is of marginal importance to him. I think I would be concentrating on what is important in a college for him beyond football and his girlfriend. What does he want to study, learn, and become. Those things will last whether or not the relationship does. It is possible that the relationship has just as much a chance of surviving if they were in separate colleges as the same college. Good luck.
Personally, I think the bottom line is this:
Girl will be in college getting lots of attention from college senior boys who prey on freshman girls and stoke their egos. Boy will be in high school devoting his free time to football and SATs.
I give this relationship 3 months. She will break up with him when she comes home for Thanksgiving. The whole football is dangerous argument will be moot.
Obviously you haven’t seen the great Thanksgiving Day rivalries. You know a bunch of overweight, out of shape guys get together and relive their youth. 
Maybe the girl is also concerned about concussions.
@mathmom doubtful - it doesn’t take following her to her school for him to avoid concussions.
I would try to get him to keep his options open. Have him continue to talk to coaches etc., but let him know that ultimately it will be his decision (provided the finances work). I would also make sure you are really listening to him and what he wants, without dismissing his very real feelings for his girlfriend. Perhaps having a discussion with the gf as well would shed some light on this or at least let her understand what he will be giving up. It may be that HE approached the gf with this idea and she is not necessarily pushing it. Maybe he doesn’t want to continue to play in college. Or maybe the gf is the one driving the bus.
With any student athlete, I think it is critical to look at the college without the sport. Would he still be happy there if he decides he doesn’t want to play anymore? Many kids get to college and sit on the bench or don’t get along with the coach and quit their sport. What happens to the scholarship and how does he like the school? Or if he is unlucky enough to get a career-ending injury.
Good luck!