Have a care.
If you keep going on and on asking him and asking him about what he's going to do, he'll get awfully fed up with it. I bet there's absolutely nothing he can possibly say to you that will convince you that he is not going to stray. The problem sounds to be with you rather than with him.
Have a good hard think about whether he is a man you can trust or not. Has he ever given you any reason not to trust him ? Do you have any evidence ? If he is trustworthy, stop bothering him about it, checking up on him etc. It's very difficult to break what sounds like a pattern , but if you keep on worrying away at him, questioning him , checking up on him, it feeds itself and you have to keep on doing it. Just make yourself stop and live with your insecurities yourself. They're yours, after all, not his.
If he's not a man you can trust, bin him.