That is the best question of the day!!
I have said many times before - If your child gets a B, your parenting gets a B.
For most of the Chinese kids, it does not really need any "study" to get good grades in regular classes. At least that is my feeling. Both our kids watch TV, play games and engage in many other activities all the time. Rarely I would see them "study". Granted that they did not get into IVY schools, I would say they did at least average per Chinese standard.
We never had to worry about DD. DS, on the other hand, did require extra attention once in a while. If he got a low grade on his 1st 9 week report card, we would always go talk to the teacher, the GC and himself. Finding out exactly where was the issue and was there any thing we could help.
Since you are paying more attention to your work. Your son might be doing this to get more attention from you. Punish him will not help any body.