Getting hard for sex

I need help! Me and my boyfriend are ready to have sex and every time I look at him and say hey should we try he goes flaccid. I have givin him a hand job to get it back and also sucked his dick to help but once sex is in the thought his erection is gone. It’s not as if he isn’t turned on or nervous, that’s what he says. He really wants to have sex and he is hard any other time. Even when he sees me, he is very attracted to me. Im not sure how I can help him and how we can have sex. Any advice?