I wouldn't worry about it too much unless he can never get it up. In which case, he has a problem. But maybe it will end up being something you guys find funny later (although I would wait until he brings it up to ever speak about it again...)
If you continue hooking up and he simply can't, then he needs to take care of it --- go to a doctor, and if that doesn't work, a shrink. If it's a long-term problem, it's either physical or emotional.