Well, it could be stress. If you are just back together he may have some anxiety, unfortunately, when anxiety causes PE, it also causes more anxiety. So first tell him to relax! Now if he is ok with it, try a cock ring. It will help him maintain an erection even after he cums. If he still cannot maintain the erection even with the ring on, then it is time for him to see a urologist. Embarrassing as it may be to see a doctor, early detection of a problem may just save the rest of his sex life.