So, by his theory - tell him to stop talking about the damned assignment. It's over with and we're in the here and now, right?
Actually, I don't think you should tell him ANYTHING. I think you see here that you can go on forever. Do you see how vicious this cycle is? He'll blame YOU for it all - helping and getting a bad mark, not helping, looking at him the wrong way or asking if he wants dinner while he's trying to work and disrupting his concentration. Then it'll be the next thing, and the next thing.....
Would you leave him if he had a different disease? Well in those cases most people don't live with the abuse that seems to come with THIS disease. Not a valid comparison in my eyes. But it shouldn't matter what he thinks anyways; it matters how you feel, and you alone. He can tell you lots of things. Really pretty empty at this point considering his actions. Anyhow, if you're so miserable and unsupportive, why does he want you around? So in his thinking, he'd actually be better off without you, no? You'd be doing him a favour by leaving.