Well, let's presume that he's not lying about being unfaithful while with you. At least for now.

The question you should ask yourself first is if you found out that they had been together in any way before you two got in a relationship, could you handle that without becoming resentful? If you could then perhaps present it to him like that. If you couldn't then you have 2 options... Forget about it because it could ruin a relationship that seems important to you and truth be told he wouldn't have been doing anything wrong even if he had hooked up with her in the past.

Or... Keep pushing and once you've heard about a previous encounter end the relationship anyhow.

We can get into the idea of full disclosure of partner's pasts but if you're not the kind of person who can be objective about that and not feel jealous or threatened then why seek it out?

Of course... This is all presuming that they aren't lying to you but frankly if you have no other reason not to trust him, then why start now?

Good luck. X