Well he played with his penis and it felt good. Let him know thats something frowned upon in public and must be done in the confines of that bathroom, shower or his bedroom. Also you need to buy him some baby wipes.
When he gets older teach him to masterbate into condoms. That way he'll get good at putting condoms on and probably won't get a girl pregnant because of his excellent masterbation experience. This paragraph was a joke.