first, he may be legally an adult, but in the eyes of a mother he's not.
He should not hear about the affair from you. Sorry, that's spiteful.
He's not the only one you have to talk to. You are here. If you need to find someone "in person" to talk to, find someone other than your son.
Using your son as "the one to talk to" about this will have no positive outcome for anyone involved. You put your son in a bad position, and that's not fair to him.
