Move him from that environment as soon as you can because the sooner you reverse the underachievement the better. Your poor son needs to know school doesn't have to be like this. Twice our DD has been in this position. Outside activities didn't help. Being in the oldest grade and having trips and graduations were of minimal if any benefit.
On the up side, both times when we were able to improve the situation, DD improved quickly. Which was good because there were signs of serious distress and depression while in the situation. She checked out academically the first time. The second time we drastically altered the academic program in place. She essentially did her own study at school. It was a bit of a help and made school just bearable to survive the last term.
I feel bad even now that we didn't move faster to change our DDs world as it was a painful time but life is better now and really she is ok and your DS will be too when you can improve his educational situation.
