I wouldn't tell him. I've slept with a few of my ex's friends or people he slept with (small community here :) And now we are back together. I have a policy of not volunteering information but I'm always prepared for the moment when it's discovered, so that I will face him without shame.
"yeah, I slept with him. I was fair game. What of it?"
"why didn't you tell me?"
"Why would i? We weren't together. It's not a big deal...it's just sex...who have you slept with since..."
And then to boost his self-esteem "Really it's no big deal, the sex was terrible anyway."
We don't need to be an open book. Be a foreign film instead, with censored subtitles.