If he's 20 and you are in your 20's, this could indeed be an issue of sex drive. Women hit their peaks in their 30's, men in their 20's. In 10 years he'll be all petered out, and you'll be wishing he was 20 again.

Maybe you should consider talking to him when you are not in the heat of the moment. Set some sort of guidelines before he is approaching you. You wouldn't want him to feel you are rejecting him daily, right? It could be hard on his ego...