Problems in Bed

So my boyfriend grew up in a very Christian family and he didn’t even start dating girls until he went to college. He lost his virginity last spring when he drank too much and blacked out, which really bothers him. Since then I’ve been the only other woman he’s been with. I love that he hasn’t slept around but I’m also concerned; when we start getting into the mood he is super excited and hard but once he actually goes in he loses his erection. I love him so much, it concerns me but doesn’t bother me to the extent it bothers him. I know it’s not because he isn’t interested in me because whenever we just make out or cuddle in bed he becomes very hard. He’s a young guy, 21, so it seems odd to me that he would have issues. Has anyone else encountered this problem? Does anyone have suggestions that may help us?