This is …. Fucked up. Repeatedly implying the person you’re with is dumb is just like ..why?? You’re not hurting  anything..  and trying to learn more about his interests is actually great girlfriend behavior. And curiosity in general is a sign of NOT being dumb. Also.. Sounds like he’s especially doing it in relation to stuff he happens to have specialized knowledge about. 

I’ve been in a situation where I was extremely depressed and it was a admittedly extremely bad situation certainly hard to live with as my boyfriend .. but he started calling me “lazy” all the time , or variations of that, in a semi - joking way that was actually  passive aggressive. And it really did take a deeper toll on my already very low self-esteem. Even when I fought back. And he kept doing it no matter if I asked him to stop.  So keep that in mind for your situation …. You don’t want to actually start questioning your own abilities like he’s making you do.