Boyfriend half clothed

I know the title sounds weird, but I mean during sex. 
My boyfriend and I have been together for over a year. Every single time we've had sex, summer through winter, he's been wearing a shirt. The only exception is in the shower. 
He's overweight and I know that he's insecure about it. It's not something we talk about often. But when we have I've made it clear that I love him the way he is and I just want him to be happy/healthy. I don't control what he eats or make him exercise.
I just want him to feel secure enough not to hide himself all the time :/. I don't know how to talk to him about this...