[masraum]

I don't remember what it's called, maybe "confirmation bias", but I'm not certain, but the more you try to convince someone that what they believe is not true, the more they believe it. You aren't going to convince him.

My suggestion is tell him that you're happy to talk to him as long as he doesn't talk about it, and if he does, you're going to call him a moron continuously until he stops.