He doesn't realize I'm sure. But you also can't teach him which is the most crappy thing about these situations. What you can do is focus on yourself and recognize he is on the fence right now most likely.
It is possible this 24 year old has a husband or something else along those lines. Dunno... does he have any family or anything that may be able to put pressure on him? Not sure if you should go that route yet - but stopping infidelity probably means your relationship will have to worsen before it gets better.
Being friendly about it gives him permission to head out the door.
"You can't reason someone out of a position they didn't reason themselves into."
