Erection question

Me and my boyfriend have been together for 9 months now (im 20, he is 23) and still havent had sex. Our problem is that his penis is hard during our foreplay and all but as soon as he trys to put it in it gets soft.. than i give him a bj and he gets hard again and when he wants to put it in the same thing happens it’s really frustrating for both of us because we dont know why this keeps happening every time we try.. he says he is not stressed also he isnt a drug user nor does he drink .. i’ve also read that it might happen if he holds it in for long but he doesnt.

Any idea??