kids at that age don't know it is wrong, all they know is that it feels good. sure somebody could have taught him or one of the other kids it, or could have been exposed to it from media sources, but sometimes kids discover these things on their own.
irregardless, teach him that these things should only be done with somebody he is deeply in love with. he needs to know that sexual things should only happen in dedicated monogamous relationships. don't make sexual things taboo or tell him it is wrong, for it could only make things worse, both in the short and long term.
also he needs to know he can get into big trouble if he is caught doing it.
Someone who will address how he got exposed to this. Who is raising the child, and is anyone protecting him from adults or older kids who might have exposed him to inappropriate sexual activity?