I've just seen your message as was about to post something similar about my son and his upcoming prom. The point about 'not finding his tribe' really hit home. It is so hard for them to fit in sometimes and I completely understand that they feel like everyone else has it sorted, or is more confident and popular. Does your DS WANT to go to Prom? If he does, then if it was me I would be trying to find a way to make that as easy/painless as possible. I think if he can go and enjoy it, then that would be a positive step. My DS is the same about not making an effort to keep in contact, make arrangements, etc. but then disappointed when he is left out, or left with nothing to do.
I'm trying to coach my DS a bit in the way he communicates with friends, so that he stands a better chance of finding out what is going on. He doesn't really like this involvement and does try to shut me out, but I figure his way hasn't really got him the result he wanted so its time to try something different.
My other thought is that he needs to keep busy. He has just come out of the end of a really long period of hard work, but in a routine where everything is planned out for him. To go from that to doing nothing can be really hard. Would he be interested in getting a job, particularly one where he is working with people, which might help improve his inter-personal skills, and may make some new friends.
I really feel for you and your son and hope he can find a way of enjoying his prom. I feel exactly the same that my DS is bright, funny, etc. but he just struggles to push himself forward and so he's not giving people the chance to see what he is about.