I think you should have stuck with telling him to go away and straighten out his life first, like you initially did! If you believe he was "cheating" on his girlfriend with you, that means you were an accessory.

Decide if you really want this two-timer (if he cheats with you and with her and with others then. . .) and if you do, tell him what you want and what will happen if he strays. But I can't see that you can trust him; I'd go with my instincts on this one.