Teach him to own his stutter, accept it and acknowledge it while denying any ill effects. When a stutterer can do that, anyone who belittles them lose all their power. A simply reply of acknowledgement and dismissal can be a very strong tool. as he gets older other things can help with this, but its tough getting kids to understand concepts like this. Edit: For his mental health and his ability to deal with the root issue never tell your son to ignore it, that will never help and will likely cause him to suppress it and build up an inability to talk about it to anyone.