I wasnt going to respond to this b/c I only try and respond to posts that I have first hand knowledge about. My oldest is only in pre-k, so I havent walked down this road yet.
HOWEVER, I can put myself in your sons shoes. My parents had crazy high expectations for me. I did well from K to about 3rd grade. At 4th grade, I began having a little bit of trouble. Instead of 95%, maybe I was at 90%. The first poor grade I brought home, I was punished. No phone, no friends, no TV. And that continued on. I spent more time being grounded, than not grounded. By the time I was in 6th grade I was barely pulling a C+. There was no way I would ask my parents for help, because I was afraid of being grounded AGAIN.
I was very intelligent. I always tested well. But my parents expectations were so far out of reach, I gave up. By the time Freshman year rolled around, I was failing out of classes left and right. My parents were furious. I gave up trying. I wasnt getting into any trouble. I was a good kid. I had a good head on my shoulders. I just didnt try in school. I didnt see what the point was.
I did pull it together my sophomore and junior years. And that was thanks to a phone call from my aunt. She gave me the words of encouragement I needed. That I wasnt getting from my parents. I did graduate. BARELY.
Had my parents been on my side when I was slipping in the 4th grade, my story would have had a different outcome. I needed some positive reinforcement. I needed some extra help. I needed some encouragement. I didnt need to be punished. I didnt need to be grounded.
So I think now is your chance to show your son that you are behind him 100%. Let him know that no matter what grade he achieves, he can come to you for help. Without fear of being punished.
Perhaps before he gets in front of the TV or the computer, you can suggest the two of you go over his spelling words for the week. Let him know you are there for him. Now is your chance to build a nice strong foundation.
Best of luck.