That’s pretty relevant to your situation here. I work in mental health, and I’m sure you’re likely already aware of this but- this sort of thing is very common with BPD, and is actually on the tamer end of what partners of pwBPD tend to go through.

It’s clear you really care about him so I won’t start with any “run for the hills” stuff, but I recommend seeking resources for people in your position. There’s a subbreddit for partners of pwBPD I’ve heard is helpful. Best of luck to you both.