Should I tell him that we are working on accelerating him? I think it would really make him feel better.
You know your kid best, but this really helped our DD--with the caveat that we always emphasized that we supported the teacher (actually except at the end of last year when it was no longer possible because the teacher was so horrible to her and us--but that's a different story). Because with our DD she would sometimes try to exploit differences between us and the teacher for her own gain--not through any malevolence, but just in the way kids do say if they get different answers from mom vs. dad--and DD vigorously explores any possible advantage, so we learned early to present a united front. Same thing with parents vs. teacher. So we would tell DD that the teacher had a lot to do and a lot of kids but that if DD behaved nicely we were sure the teacher would try to do what they could to accommodate her, and that we were trying to help the teacher understand how much DD would like advanced math (which we were). It really helped her to know that we understood her and were trying to do what we could. Good luck!
