You really can't change the mind of people like that. Don't aggravate yourself trying to convince him otherwise. Who cares what he thinks. Really. In the grand scheme of things, he's just a misinformed person who is argumentative. So he doesn't believe your diagnosis is correct; don't give him the satisfaction of thinking you care what he thinks. Whenever I run into someone like that, I usually just smile and say "You could be right," then change the topic. I'm not saying they ARE right (and in my head I'm probably thinking they are an idiot) but they can be smug and all proud of themselves and move on to something else. It's hard for them to argue if they think they've "won."