He's nervous. In cases like these, it's as simple as taking your time and letting his sex drive kick into high gear. Remind him that his penis is beautiful and special. He's drunk. Drinking delays reactions and mutes nerves, so his penis won't get excited the way it's supposed to.
I lose my erection during penetrative sex | Life and style | The Guardian
Men love sex. This is a fact that is widely known. Unfortunately, not all men are alike and, even though most men do enjoy having sex, some men experience some problems when they participate in sexual intercourse. Testosterone is a term that has gained a lot of recognition in the last few years.
It will happen to every man at some point. Men usually describe the first time they could not have full erection or they lost their erection during sexual intercourse as a particularly awkard moment that they could never have thought would ever happen to them. Then usually various thoughts cross their mind: "I have a serious health problem"; "Why happen to me?