bluemagic I've seen you post numerous times now about your DS's frustration with math. My eldest DS went though much the same thing once he hit AP Calc. and through Calc. II...at which point he figured out the majority of his problem, it was simply that while he always understood the CONCEPT, he hadn't been applying enough repetition. Once he actually worked enough problems through, he found his success increased considerably. According to him (Now having been through Calc. III as well) It's not enough to simply understand the concept, you need to PRACTICE APPLYING IT in order to be successful on the tests. While I can't assure you that's your son's problem, it's worth considering.
Relating to the social aspect, just try to involve him in activities where those of like mind are. Summer band camps are an opportunity to start fresh with new faces as well as being around those of like mind.
I've found that most Freshman / Soph. eventually come to the realization that they're going to have to put some serious work into instrument / vocal practice in order to play with the "big boys" Either then buck up or they're simply not interested enough....and that's okay too. We all need to sort out what is really important to us every so often, it lets us focus on those things that ARE our priorities.
Just saw this. I do agree that more butt in seat time practicing would probably help him a lot. It's what I had him do for his semester final. So far second semester is going better, but so far it's only logs and graphing and both topics are fairly easy.