Has your son felt more challenged at things which were once 'easy' for him? I ask because this article may help you:
http://nymag.com/news/features/27840/
This focuses on the reluctance of "smart" kids to try new things or accept challenges. This may or may not be part of the problem.
The other thing that stands out to me is that truly, from your description, he sounds depressed. Perhaps you would try talking to the school counselor; the counselor may be able to connect with your son in a very neutral way and find out if something serious like bullying is going on. Has something significant changed in your lives? (Shift in family structure, job loss, move or foreclosed home? Family member deployed?)Kids can worry about far more than we might suspect, so a school counselor might help uncover what's getting him so upset.
I hope you get the answers you seek and that he feels better about things soon. There's also a good book too, which I'll recommend (if Peg M. hasn't beaten me to it:) ) "How to Talk So Kids Will Listen...And How to Listen So Kids Will Talk" by Faber and Mazlish. It's right up this problem's alley...