As strange as it sounds, the distance may strengthen your relationship, especially as he goes through treatment. Having a child together binds you for life whether married or not and all of us benefit from learning the specific communication and relationship skills required for BPD relationships.
I would also trust and listen to your family. They know your story better than we do, and they are the ones who must bear the brunt of any discord and conflict that accompany BPD relationships, not to mention the heart ache they feel for their young nephew.
I'd like to add to that first paragraph. It is not unusual for things to
appear better with distance apart. BPD is a PD that is most impacted by the type of relationship. From a distance many may think the person is "a little off". Get closer, get back into a committed or obligated relationship — when the person hasn't really addressed the deep issues — and that's when the wheels start falling off the relationship wagon all over again, especially if you don't have strong boundaries clearly set. A few months of better behaviors is not proof of long term recovery.
Henry Cloud wrote an excellent book,
Boundaries.
Boundaries are for you, not him. You already know you can't tell him what to do or not do. You can't force him to do or not do something, your power is in your
response. What can and does work (though there are limits) is something like this... .
"If you do or don't do ___ then I will do or not do ___."
Examples:
If you start blocking me from our kids...
...then I will enforce the parenting schedule, in court if that's what it takes.
If you want extra time for ___...
...then I may allow it but with a trade for equivalent time for ___.
When done right "if... then... " is powerful. It took me years to figure how to make boundaries such as these.