You should see if he is willing to take supplements. This has happened to me twice before. Both guys had not had sex in 2 or more years.
Guy#1 - He said he was just nervous, but I was not certain so I purchased maca supplements which he drank daily for a week. The next time we tried, he was able to maintain an erection for a long time. We did not have any problems after that, but he may have continued taking the maca in smaller dosages, not sure.
Guy #2 - He couldn't admit that there was something wrong with his penis and wasn't really willing to take supplements. He was taking medicine for high blood pressure which I know effects libido. I tried to have a relationship with him that was not centered around sex, but after five months I had enough. He did let me make him a maca smoothie once and I noticed he held an erection for longer and seemed more excited.