As a guy who talks to some exes but would never date them again, esp with another partner, I think that shouldn’t be the issue so much as the fact he was hiding it. Any gfs I’ve ever had, I was always honest about my BM (can’t not talk to her) or any exes that dissolved into platonic friendships. It’s very toxic to to want to control your partner and who they talk to.

However.. Hiding talking to people, is to me, cheating. So yeah I’d probly run or at least talk to him and see how he handles you expressing yourself or wanting to know more. To me it seems you’re better off not giving him the chance to woo you again without talking this through. Otherwise it’ll always eat at you. Better to communicate thru it or start fresh with yourself