Should I tell him?

I started seeing this guy, let's call him Michael, for about a month. I genuinely like him and feel like we could be in a relationship. We aren't officially dating yet, but we won't be seeing anyone else.

I have a friend that was also my fwb for a short period of time. Before anyone says anything, Michael knows everything about my friendship with him and we are not jealous people so we believe whatever is in the past is in the past. Just to clarify me and this guy were friends for 3 years, and about a year ago we had sex prob about 2 months or so.

This friend of mine asked to catch up for a coffee, I agreed. Michael is aware of all this. Then we were just talking normally, totally platonic. And I told him about me seeing someone now. He seemed happy for me.

When he was about to drop me off, in his car, he came closer and asked, "One last time before you guys become official?"

I said, "No. Even if we aren't official, I feel wrong if I slept with someone else. I don't want to do this because I'm serious about Michael."

He insisted that technically it's not cheating since we aren't official. And he said, "Just one last day and after this so Michael can have you all to himself"

He started flirting with me saying that I'm really cute or "from what I see I don't think you're that invested in him"

I got really annoyed and left. But now I don't know if I'm supposed to tell Michael about what happened. This has been bugging me the past couple days. I feel guilty. What should I do?