Should I get back together with my ex?

I broke up with my ex about two and a half months ago. I was trying to get him back (but I wasn’t trying really hard, didn’t want to look overwhelming) and I never seemed to get positive feedback from him.

Then suddenly one day he asks me if I want to start it all over again. Of course I replied yes because I wanted to be with him – that was two days ago. Yesterday I asked him if he had been meeting someone else in the meantime and he replied yes. I asked if I knew this person to which he replied yes and added that he didn’t want to reveal the identity of that person.

What should I do? I’m always straightforward and honest in relationships (or at least I try) and I don’t like hiding stuff like that from each other.

Am I being selfish wanting to know who this person is or should he tell me?