I would agree with him if it "doesn't matter" like whether he is smarter in history (or even whether his IQ is larger). Let him be "smarter" if he wants. That doesn't change actions and what is done. I went to my aunt's MENSA group, a party they were having, and I was amused because there were "regular" people there not working or working at low paying jobs but getting their kicks feeling they were more "understood" by these other people with high IQ test scores and were special in some way because of it.

You say you're more interested in medical interests so I would persue that arena on your own (and learning how to work with the kids, etc.) and cheerfully agree with what he says about history and agree that you're not as good at history as he is, even let him be overall "smarter" (whatever that means :-) than you are. Just because he feels/thinks one way does not have any bearing on the reality of the situation, your honors program, etc. You know you're smart enough to suit you :-) and that's all that matters there.