My husband can only get a full erection once a night and cannot last long.

My husband is in his late thirties,well our sex life is not that good at all. He can only get rock hard once a night. Once he is hard it does not last no longer than five minutes.I never even have time to get off. Even if I ride him.Once he cums it is over. I have hit my prime and sex crazy. If we even try to go for round two,his penis is like a flat tire. Can someone please tell me what to do to fix this problem.