Wow... reading your post brought back flashbacks!! I was that child. I had been in the gifted program since kindergarten and skipped the 4th grade. But when highschool rolled around, it was a different story. College... whew! I lost an 4 year scolarship because I had a 1.6 GPA my first semmester!
There were two reasons (I wasn't aware of at the time but I have since come to realize) for my underacheivment.
1. I had NO idea how to study. I breezed through elementary school without having to crack a book. By the time I got to high school and had to study for subjects that challenged me, well, I just had no idea how to study or the discipline to do so. In grade school my teachers never challenged me because everything came so easily. When something didn't, I had no idea how to deal with it. Which leads me to point #2.
2. I developed a huge fear of failure. Because everything always came so easy, I was CRUSHED when I got my first B in 7th grade. I gave up after that because I was so devastated about not being perfect, I realized I'd rather either give up ("Well, I just hate so and so so I don't want to be good at it") or half*** it (since that way I could say "Yeah, haha, I totally bombed that class because I didn't do homeword but 5 times all year") than put my effort into it and not get it perfect. This has haunted me ever since, with many things I've attempted.
Don't have any advice, but I just wanted to give you some possible insight into your son while it was still early before he ended up like I did. I changed majors 3 times in college and finally dropped out because I was so overwhelmed with the above challenges.
Hope things get better, and hope I've helped a bit! If you find a way to help him, please let me know!