Why can he not get off!?

My boyfriend and I have been together for a few months now, and our relationship is great. However, when we have sex more times than not he is unable to get off and after we go for a while and try different positions, dirty talk, etc. he ends up just pulling out and finishing himself on me or in my mouth. We have talked about it and I have asked him if there is anything else that I could do/that I'm not doing right and he's told me no. He says it feels good and he enjoys it, he just can't finish, he doesn't know why. Does anybody have any advice or why this is happening?