Just a thought, but what would happen if you the parents role-played gently regarding perfectionism issues. Imho, the perfectionism and seeing things black/white are related. The anxiety of not finishing, etc etc
Role play a lot of what-ifs with him so that he is prepared to see unfairness, that it is ok to not get a a perfect mark -- e.g. "oh horrors" what if I got that *all* wrong -- oh well, at least I think I'll learn something new, etc etc So if he responds to the role-playing. You know he is when he tells you that he thought about something differently.
Also try, as the parent, showing very clearly (slightly exaggerated way) how you're learning not to be a perfectionist. So when you make a mistake, so how disappointed you feel, then show how you will re-assure yourself that it is ok and how you'll move on from that... etc.
He needs to experience small successes and to see role-modeled to him ways to deal with knowing what *should/can* be versus what reality really looks like -- that is, full of mistakes and issues -- but "that is ok"
don't know if that helps, best wishes!
