Taking away privileges can provide your son with the time to study and get his life back in balance with the priority being school since it determines his future. If you are punishing him, it may just cause a different problem. Does he fully understand what his grades mean to society, and how he can earn more money for the car he wants, things he wants, etc for the rest of his life? IF he attends school with the attitude of pleasing you, or just getting his priveleges back, it won't solve your problem!!! If he sees school connected to HIS goals, your problem can be solved for good!
This is a great opportunity to show colleges that he failed 9th grade and then turned it around to get A's and B's. Drastic improvement shows great character and determination that are part of all our great people in history. So, in that respect, your son is off to a great start.
Show him with that you want him to succeed for HIM and not just to please you or he may just become an expert at "getting by" or "hiding" things you disapprove of, instead of making great decisions for his life, for HIM to be successful no matter who is around.
Good luck!!!
B.