I know when he is in this reactive space, he is not capable of accountability for his being hurtful.
Hello and welcome to the family! I'm so sorry you're in this position and I walked the same path with my BPD daughter, who's also now 26. Her symptoms started even earlier than your son.
Let's start at the line I quoted above- that he's not capable of accountability. That's completely false and it is single-handedly the biggest "BPD excuse" that you'll ever hear.
"It's not my fault, it's my BPD!"
Yet when your son stops at a convenience store to grab his favorite drink, does he abuse the cashier? Does he explode on neighbors and co-workers, relatives and friends?
No, probably not...at least in the way you experience it. Most of my relatives refused to believe my daughter was even sick, because they saw a completely different person.
You cannot save your son and you cannot convince him to change or take therapy seriously. All of that is up to him and him alone, nobody can convince him. On the other hand, you are responsible for you and you can hold him accountable every single time he speaks to you in an unpleasant manner.
How? You listen with empathy, you try to help him calm down, and when that doesn't work you walk away or hang up the phone. If he continues (which he will), you ignore him and put even more distance from the situation. If he wants to rage, so be it, he can do that with someone else. You can decide though that you're no longer going to be front and center for his abuse.
Now, this is incredibly difficult and things will get worse before they get better. That's because BPDs are master manipulators and they lash out when they don't get their way. That's fine, let him do what he must, and continue to be there for him when he's kind to you.
But when he's not, you must take a step back and speak about your own mental health, that the conversation is toxic and you need some space from it to think.