He can’t finish???

So my boyfriend is about 158lbs and kinda short, so he has a little chin on his stomach right? Well we’ll be a minute or so into having sex and he just lays down on top of me. He says it’s because he’s too tired to keep going. I offer to switch and I’ll be on top but then he just lays there until he rolls off. He then rolls over and just goes to sleep. I’ve asked him about it and he says that he just gets tired, but after he rolls over he won’t talk to me. Am I doing something wrong? Do I just not turn him on enough to keep going? Help