Seekinghelp.....you will oprobably think that i am splitting hairs----but, what you describe as a boundary for yourself is, realy, a Rule that you expect him to live by (in the strictest sense).
The actual boundary, for yourself, in this, would be----"I will not be in a relationship where my partner maintains an unhealthy connection with an ex. And/or...I will make it clear to my partner that I do not approve of and will not put up with that kind of thing".
It will be up to him if he wants to continue to make that kind of asssociation with the ex. However---the big however--it will be up to you to enforce your own boundary.
Others may Choose to honor our wishes---but, they cannot be expected to enforce our own boundaries that we have made to protect ourselves.