i dont really have any adivce other than ask questions. is he shy? do you talk about what is going on at school? maybe there is a girl?
a famous story about my hubby when he was in school - he was doing homework but failing a class. Eventually his mom and dad sat him down and said 'you are doing your homework right? 'he sayd 'yes' and they said 'why arent you turning it in?" and he answers "well, you know that girl i like? well her locker is on the other end of the hall and if i walk her to her locker i dont have time to get back to mine and get my homework for that class"
it's funny NOW but at the time his parents were NOT ammused! teenagers have messed up logic and sometimes i think we just have to try to think like they think!
also, if it helps, my sister was one of those super smart never had to study types and when she hit highschool she suddenly started dropping in grades. Mostly because she felt like if she didnt have time to work on something and make it an A she would rather get a zero. (again with the messed up logic). taking away things didnt work with her either
maybe just a really cute tutor? sometimes kids are inspired most by peer pressure! if his friends are studying and getting good grades (or someone he looks up to) then he might also want to do that.
good luck and let us know what you find out!!