HE DOESN'T CUM!!!! 😡😡😡

So my boyfriend and I have had great sex!! And like he has came before like in the past. But recently he hasn't, he says he doesn't know why he can't. We both get super frustrated because he'll literally go from being hard to not being hard at all... honestly it offends me because I feel like it's me. He says I'm not the reason and I'm "hot" and "sexy" and stuff but it doesn't seem like I am because of this problem. I've never had this problem with my ex boyfriend.. does anyone know or have a clue on why he just doesn't cum??!?! I NEED ANSWERS!!