You tell your son the truth, and ask him if he wants any DETAILS about it, he should go ask his father.
"Yes, he did, son. I'm sorry you have had to hear that, but I won't ever lie to you. Your father loves you, very much, as do I, but yes, he had an affair and that's the reason our marriage has gone thru this. If you want any details about it, I suggest you ask your father. This is NOT your fault."
