I'd like to echo the thank you to Caron! I still wonder if it was a sort of non-breakup-breakup (ala "Friends" TV show, "we were on a break!"). Separate colleges could IMO lead to that even between people who love each other.
I agree it might be none of our business, but just jumping off, now I'm confused re: the keys. I thought "friends" (which I took to mean local friends in Roseville/Rocklin area) took them away, but it was actually his girlfriend? So was it the girlfriend that his Mom talked to and told to give him back the keys? Or were his other friends also with him at her place? I mean hopefully she was a friend as well as romantic girlfriend, but that really confuses me. It may not even matter, but like you PoirotsGirlFriday I'm trying to understand his emotional state that night.
Total speculation and I hope this is ok, but I can see him reacting very differently if, for example, he went to break up with her and she went "oh thank goodness" as opposed to "please let's just give each other some space" as opposed to "no, no, no, don't break up with me, I love you!" I can also see the key thing really differently in that case. "You're acting weird, give me your keys" from a group of friends is really different from "no don't leave, give me your keys" from a girlfriend you're breaking up with.
It's been so long now I'm not sure if these answers even help with finding him. They might though. It might help with figuring out, was he likely to take off running, hide, end up in a mental hospital, or potentially commit suicide (boy did that hurt to type)...? Was he in a decent state of mind to take basic care of himself after the accident - i.e. finding food, water, safe or semi-safe place to stay? If so does that mean he's surviving but lost and dazed somewhere? Is he likely to return to an old friend's house and beg them to conceal his identity for now? If not, well... I really don't want to go there yet.