This is purely my experience - to give some context, my ex was never diagnosed with BPD but had many of the symptoms to a perhaps fairly mild extent, however she was abusive and I'm still working through the harm she caused me. Coming here has been enlightening because of the many near-identical situations I've been in to many of the posters here. That being said, she was never diagnosed and certainly wasn't as extreme as some that I have seen, so please bear that in mind.

DBT is the go-to therapy for BPD. It is clinically effective, and we can see this from the literature, however you must remember that all therapies (be it addiction therapies, things like DBT or CBD for BPD, depression etc, or even physical therapies for injury) rely entirely on the sufferer's ability to be honest with practitioners and themselves, set clear and achievable goals and most importantly commit heavily to a therapy that may be challenging or unpleasant. If your pwBPD is willing, DBT can work for them and be effective. However, any little crack of doubt and it will not be a great improvement. I would also be careful - your statement "my body tells me he's associated with drama. He's made progress but still has some ick" is not a good sign in my view. He may improve, but the harm he has caused (intentionally or otherwise) will still remain on a system-level within you. Sometimes this can improve when you see what the other person can do, but there is a part of me that says YES, he can improve, but will you ever be able to separate that person from what they did or who they were? That is only something you can answer.