What do I do?

What am I supposed to do when my boyfriend has a hard time “keeping it up”? I understand it happens after a night of drinking usually that’s fine but this started getting worse when we were TTC 7 months ago. It started off fine, all hot and heavy then started fading away. I thought maybe he was just getting tired of doing it so often. He’s 24 and I’m 21 so it’s not an extremely common thing for people our age. We ended up conceiving and he is so excited to be a dad but now we don’t ever have sex. We tried the other night, and he just kept saying how nervous he was, so I told him I wanted the lights off but he wanted them on and we just ended up stopping and he got awkward and started making fun of his size and I just went to bed. Last night I told him that we HAVE to talk about it because something is clearly going on with the lack of sex and he told me he doesn’t know what’s happening but it has a hard time staying up. He says this happened with his first gf, which makes sense to me because she would brag about other men but I don’t do that, so why is this happening to me? My ex was an alcoholic and could still get up so I’m just really confused and hurt.