Husband worried about sex life?

So a back story, we are married and still very young. Our sex life before I got pregnant, multiple times a week, now it's maybe once a week or once every two weeks.

So I'm 31 weeks, and in sure most of you understand how pregnancy can drain you. I'm also having a hard time with my athsma and allergies, so sex is really the last thing on my mind. My husband tries almost everyday to turn me on but I'm just not into it. It has nothing to do with him, I'm just always tired. He's put on some weight in the last few months and he seems to think I don't want to have sex because i don't find him attractive. I try explaining that it's not him and I still find him attractive but it's hard to explain and it just ends in a fight. I'm sick of fighting about sex almost everyday but he just keeps trying. I know it must be hard get turned down all the time but I don't know What to say to him so he can understand. And hopefully not try everyday.

Has anyone else gone through this? What did you do?