I had an ex who wouldn't accept that I had chosen another guy over him. He wouldn't leave me alone, especially during the initial dating period with the new guy, telling me the other guy was not good for me, I should get back with him, etc, etc, doing what he could to sabotage us. One early morning he actually jimmied my front door open while I slept, and I woke up to him cuddling me in bed! That was the last straw.
I told my new BF about the situation, so he'd be aware if the 3 of us crossed paths anywhere. We both wanted my BF to get to know my gay friends after he moved to live with me, and we became partners, but I told him the ex was one guy I didn't want us to have anything to do with, not even answer his calls, he was poison. He agreed and we both kept our distance from him, with the ex fortunately moving away soon afterwards.
My partner didn't need to tell me to keep away from my ex, I just knew to do it myself, because of his clingy, if not stalking & subversive behavior. Maybe the OP's BF should know to do the same thing with his ex, too, without being asked. And if he doesn't, then the OP should explain that the present arrangement is totally unacceptable.