Thanks for getting back to me. Sexual play between boys is not uncommon, but for the older one to want to indulge in this sort ofbehavior with someone four years younger, is somewhat worrying, especially since there was a threat involved if the younger boy did
I think you need to ensure that you explain clearly to your son that his kind of behavior is not at all acceptable, and explain the approriateness of sex between adults.
Be objective, stick to the facts, do not be accusing but tell him he must never do this again. Once you have done so, leave it at that
So long as he does not repeat this behavior in an obsessive way, I don't think that there is a real problem, but if it becomes apparent that he is doing it again, professional help is indicated.
As far is the younger boy is concerned, it is approriate to make sure that he understands that what happened is bad, and if anybody at all wants tou touch his genitals or have him touch theirs, he must refuse and tell an adult at once. It is important though that this does not get blown up out of all proportion, so some care is needed.