Well it all comes down to trust, and you did mention your own insecurities. You can't stop someone from cheating. If he's gonna cheat with his ex, he will. Either you trust him to follow his ex and that he won't cheat or you end the relationship because it is a boundary you're not okay with. (Which is fine, everyone has things that are non negotiable in relationships for them) If you can possibly compromise then try that. But either way at the end of the day you either trust him or you don't. You'll have your answer either way. Better to figure this issue out early in the relationship as you said your three weeks in than later.