Like what would it take?
That's big territory. What's your situation?
Yes that is a big piece of real estate. Maybe a little more specific would help with some feedback.
Does he see It as cheating or does he try and justify It to you/himself that you're not in a committed relationship so it's okay (in his eyes)? IS he committed to your relationship or is he seemingly just along for the ride? Are you together with an unspoken agreement, and he feels he can use that window of opportunity?
Understand that takes an act of will on his part, to choose to not pursue others, to consciously remove himself from situations that might provide opportunity to stray. Understand, he makes this choice every time it happens and only he can choose otherwise. His behavior (like everyone else) is 100% his responsibility.
What you CAN do is make some choices for yourself and your own well-being. Decide what you want and what you can change for yourself, and move in that direction.