speaking as someone who recently had this happen to him for the first time (and every time since then)

this can be a topic of extreme embarrassment and frustration. especially like in my case.

I had been in an abusive relationship with a woman for 7+ years. in the first two months we might have had sex maybe six times, and none thereafter for the rest of the 7 years (when the abuse started). up until this point I had been proud of my ability to have an erection on demand (I am 52 now, and the discovery was in may of this year) and able to maintain it for as long as I wanted. well after 7 years of no sex (except for masturbation) I finally got laid in May. imagine my chagrin when my equipment failed me in the middle of intercourse that I was highly desirous of. to put it simply, it made me feel like I was no longer a man. it was very embarrassing, and frustrating. fortunately she was understanding, and let me finish her off orally.

equipment failure in the middle of inter course is a very sensitive subject to bring up with a man. however depending on his views of sex, and his open-mindedness a very gentle approach to an extremely open and understanding communication to discuss this I believe would be the best tactic.

I know if I had an understanding concerned lover (no I haven't had any sex since the event which lasted for 5 or 6 tries) as hard as it will be for both of us to discuss this it would go a long way towards removing any negative feelings I might have because of it.

again, this is my own experience, others may vary.