Are you sure he'd leave? Some guys (not all, mind you) either have a fantasy about this, or feel that 'it's not cheating as long as it's with another woman'. Stupid, I know, but a lot of guys I know have said that. Maybe yours is one of them? Why don't you start a conversation with him about a similar topic, and ask him hypothetically? See how he feels, and if he seems like he might be okay with it, ask him about it later.