He's doing pretty well then, I would say better than most. Most intercourse sessions last on average, at least statistically, around 30-45 min. You have to take care of your health down there for obvious reasons so just be honest and say to him it starts to hurt and if he is serious about how you feel both emotionally and physically, he won't be hurt in his feelings. In fact, his ego might be stroked a bit because he'll think wow I really am too much for her and you can even say that to him, something like "John, you really know how to please me, but I think you are almost too much for me and it is starting to hurt down there, do you mind finishing off for me?". Make him feel good about himself and also make sure you let him know that it is important to you and not just a matter of having fun.