Hi Jzc, joining Woolspinner in welcoming you!
Regarding getting your husband into treatment, I have some thoughts here since I have recently gotten a tentative agreement from my wife to try DBT (sharing a link below in case you're not yet familiar with DBT, which is seen as the most recognized method of treatment for people with BPD). I'm sure the article Wools shared will echo some of what I'll say.
I personally would advise against ultimatums. I know at least one member here whose partner went into treatment in the context of restraining order and they seem to making progress, so I won't discount it completely. I have tried them myself. But while ultimatums such as divorce or limited access to children may indeed prompt someone to seek help, if the motivation to seek help is not genuine and internalized, that person is likely to give up after a short period of time.
My wife agreed to DBT in the context that the problems in our marriage came from both of us and that I would try it too if I could. I agreed to make every effort to get myself into the program, too, though I expressed a combination of her in DBT and me in therapy might be better (I'm already in therapy, though she doesn't know this). The key point is that you bring it up in the context of wanting to help the relationship, not "fix him". No one wants to hear or believe that they are sick. It's the difference between telling someone they are fat and you'll leave them if they don't lose weight and that person discovering for themselves that they need to do something about their weight. The person in the second situation is much more like to make a commitment.
I will also share a link to great video about talking to people who are not aware they are mentally ill to help you get in your husband's mindset.
My suggestion is that you and your husband meet with your counselor together in the context of wanting to help the relationship, which is sincere. That's if your counselor is willing to do this; if not, perhaps you should find another who is connected to your nearest available DBT program. In the meeting, you can describe your husband's behaviors without talking about BPD, since these are facts. Your husband can also voice his opinions about the relationship problems, even though they will likely be distortions of the reality. If the option comes up for you to join DBT as well, I suggest you consider it as I am. Even though it's not fair we should have to do the work, too, DBT is helpful to anyone in my opinion and if the results are good for our spouses, I think it's worth the investment.
Have you spoken about the BPD to your counselor, and do they have any professional training about the illness that you know of?
~ROE
https://www.borderlinepersonalitydisorder.com/what-is-BPD/treating-BPD/https://www.youtube.com/watch?v=NXxytf6kfPM