Need help..or advice?

My boyfriend and I have been dating for 3 years and we are both finally comfortable enough to have sex. We’ve sexted before but just recently I’ve started giving him head and we’ve had sex once. I’m starting to feel like I’m doing something wrong. Every time we do it he gets to the point of almost cumming but ends up not being able to. He said that I’m doing everything right he doesn’t know why he can’t. Any suggestions on why?