Is it possible that people at the school have said something to the effect that he "can't possibly read that well" or that "kids this age don't read or write this well"?
I know some people on the board have had problems with the kiddos hiding their reading and writing skills because somebody told them they really couldn't do what they were obviously doing.
The other side of that comes from the other kids in the class -- if he's being made fun of because he reads too fast (happened to me, happened to DS, may be happening to DD), or because he writes stuff the other kids can't read, he may just decide it's not worth the effort and shut it down.
