That would drive me crazy as well. I saw a dr last year who told me that unless I had photos or witnesses, I couldn't say I was abused when I was young based on memories coming up and symptoms. In my mind I was like no, i can't prove it in court, I can believe anything I know to be true, whether I can prove it to someone else or not. I ended up switching drs, based on this issue and quite a few others and have been happy.
you said you hadn't brought up the issues with him, but is there anything he's said about how you tell the difference between thoughts, others' thoughts and psychosis? If he's sure it's not dissociation, he would (logically) have to have a reason behind those beliefs. If it's that he doesn't believe in alters, that would be a big issue for me...