Should I tell him I slept with someone

So basically my fiancé and baby daddy to two, cheated on me in November. I moved on and stayed with him but then I broke up with him again because I couldn’t handle it. Since everything happened we’ve been under the same roof and acting like a couple (toxic I know), it’s hard not to when you share kids and a house together.

Anyways, I went out last night and slept with an old flame. We are technically not together but I don’t know if I should tell him if I ever get back with him..