Maybe you can get him an out-of-school "mentor" of some sort?

http://www.ciswa.org/

I had trouble when I was that age and my French teacher suggested my brother help me but my stepmother knew that would never work in a million years so got me an upperclasswoman and I really blossomed under the individual attention and was able to do better. I had anxiety too and was fiercely sure I didn't need outside help and resented it but the girl my mother hired was really nice/kind and my grades and self-esteem went up together.