Hi Kingston, welcome to the group

"Victim" identity is common for people with BPD (pwBPD). It is difficult for them to accept responsibility or blame, and often, they shift blame to others and to external situations. Your son's way of approaching life as a victim sounds very familiar.
You mention he has bipolar, was he diagnosed? If so, does he accept his diagnosis?
Given that he has bipolar, I wonder if there's something more going on that leads you to suspect BPD, either instead of or in addition to bipolar?
It sounds like he's been compliant with his DBT program, though, which is a plus.
Does he still live with you?
I have always been a fixer, do more that is asked, gets VERY anxious about the behaviour of others.
That's also familiar. There's what other people do, which can be problematic, yet at the end of the day, we have to deal with our own responses to others.
Sounds like you've tried "fixing" your son in the past -- is that accurate?
Learning new tools and skills to manage ourselves, and our own anxieties, is a big part of the culture here. It's heartbreaking when we can't help the ones we love, yet, when we help ourselves, that positions us for a different kind of success in our relationships.
Looking forward to hearing more from you;
kells76