I was the same way. You need to find some way to challenge him. I couldn't (and still couldn't) give a flying shit about school because it was boring, and for the most part, useless. The only shit I took intrest in was shit I liked, and at 5th grade there wasn't much.
I actually flunked the 6th grade. Not because I'm stupid, not because it was too hard, but because it was boring and I felt it to be useless - so I didn't do shit. Bribes don't do shit, taking stuff away doesn't do shit. I was always smarter than my mom, who once tried to deactivate the TV with a lock. I simply "hotwired" our TV so that I could watch it.
Have you tried having him take an offical IQ test? I'll bet that he isn't stupid. I'll bet that he just doesn't give a shit because he isn't being challenged. He doesn't take an intrest because there is nothing intresting to him.