thank you for your advice.
i know there are culture difference and generation gap between my son and me. while most chinese parents would like their children study hard for their future, the children emphasize more on personal interest (freedom). youth got higher scores in computer games and sports would get more respects from peer (peer pressure). I have tried not to talk too often about study thinking he may feel annoied. so far he is till fine in his study, still can manage to make average in scores (top scores in maths), but he is doing worse and worse. He is too popular, has too many friends including some aggressive girls. some of his friends seem wild. i can't tell him you cannot play with who and who. i tried to ask like 'Tom and John seem wild, why his parents allow him to hang-out everywhere until midnight'.
what i do now is try to keep him home, i even allow him to play games for hours a day, if he went out, try to find out where he was.
I don't know how many more years the struggle between my son and me would continue.