SO uninterested in sex

I'm about 27 weeks pregnant and my boyfriend and I have gone about two months and only had sex one time. We used to do it every day of the week sometimes. I try to start things but he just seems uninterested. He does work 2nd shift and is often tired. But when he's awake I try to shower together or flirt and get things started and nothing. He says it's not because of my pregnant belly or the way I look pregnant. But I can't help but think that my weight gain and big belly has something to do with it. Any advice?!