Should I tell my exboyfriend I slept with someone else?

I had been dating a guy for a few months, and we broke up about a week and a half ago. The initial breakup conversation was very brief and took place online.

He wants to get together tonight and discuss the breakup, which is okay with me—I understand if he feels the need for closure, or wants more information.

However, I think he might also suggest getting back together, which I have no desire to do. I just don’t feel the same way about him that he feels about me.

Since we broke up, I slept with someone else. It was a casual fling and will almost certainly not happen again.

Part of me is thinking that if I tell him I slept with someone else so soon after breaking up, he’ll stop wanting to get back together. Or would that just be mean? In general I prefer total honesty, so I don’t know how to navigate this one…