Okay guys

So I just got back with my ex any way he was my first and in the 2ish months we weren't together I was with someone else which I'm guessing he thinks I was with another guy cause he jokingly said something bout him being the best I have ever been with (which its true lord is it true) but I'm wondering do I tell him I slept with another guy or just let him bring it up and if he asks tell him? I regret the second guy I was with cause the whole time I wanted it to be my ex the one I'm back with now.