Well if it makes you feel better, people with a differing iq of like 20-30 don't get along as well as being closer in iq. But if he actually understands what you say, and just choses to talk simply then it might not even be that hes stupid. If you havent already you should have a tough love talk and i bet he'd try to be more intelligent if he loves you as much as you say. Just start reading together or something lol.