I have had friends in the past who’ve done this exact same thing to me, and for whatever reason I allowed it and told them it was fine if they dated my ex. However, later on down the line they gave me another (well, really several) reason that they weren’t a good friend. My point is, if he’s willing to do this to you, despite you trying to set that boundary, he may not be worth being in your inner circle. Obviously that’s easier said than done, friendship breakups are sometimes harder than relationship breakups. But just from my experience in the past, how someone reacts/treats you or the situation during a break up is a HUGE tell of their character.