Health risks? None whatsoever. In fact, it's good for both phsyical and mental health. It's good for the heart as It lowers blood pressure and it releases (for empaths) oxytocin, a chemical that helps you relax. :)
Thank you everyone for your responses. So it's normal for a child of that age? And there are no health risks?
Yes it's perfectly normal and no, there are no health risks. Just talk to him about it and let him know that it's normal. Kids get all sorts of crazy ideas in their heads if parents don't explain things to them as they are.
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.