Maybe this is too simplistic of a suggestion, but are you somehow feeding the negativity? Are you responding strongly to it? At the beginning, did you try to ignore it and redirect his attention? Kids pick up on what works to get them attention and he may have hit a jackpot here with attention or responses from parents, especially if it's not like that with other adults at school. If you tried this and this is a drop in a bucket suggestion my apologies. I've just noticed with my daughter when I respond strongly she amplifies her complaints but when I'm matter of fact, redirect, and focus on good or other things she does, too. Again this may be simplifying your situation too much if you're already seeking therapy. Good luck.