Hi RobertX;
Your intuition is right on here:
Should we back down and agree to his version of events? Wont that do more harm than good in the long run?
I think you understand that "buying" a moment of peace in the short term, through "making the conflict stop" by agreeing to his perceptions, is ultimately unhealthy in the long run. It will be difficult and uncomfortable in the moment to hold on to your integrity in a balanced way, yet I suspect you and others here know that agreeing with his perceptions, even if you don't really mean it, will come back to bite you. He may zoom in on that agreement and hold on to it tenaciously, which would then just create another conflict to argue about -- the new conflict of "you're liars, why did you say you agreed that X happened when you thought it didn't, now I can't trust you".
It also sounds like he's getting something out of the prolonged "discussion" / validation of his feelings. You are doing the best you can by validating that Yes, it's true he feels that way, and yes, if you felt that way, you also would be distressed (or whatever -- I'm just imagining a typical scenario). Yet he wants to extend that interaction for some reason, and like you're saying:
he wants us to defend ourselves from his attacks...he appears to love the combat
Couple of ideas for additional tools to add to your toolbelt:
First of all, you are human, and are limited. You don't have infinite energy and attention for his needs... even if he were "normal". So, at some point, you need the interaction to be done.
Also, if the prolonged interaction is something he really wants, then he can "do the work" to have a topic to discuss... not your job.
Putting those together:
A new thing to try could be the "what new information do you have about this topic" move. pwBPD often rehash the same themes over and over again, wearing down participants with repetition of the same grievances (about the past, about your wrongdoing, whatever). Consider "passing the work back to him" by saying something like "what new information do you have to share with me about X" (or something more in your own words). You can be ready to listen to new information and if there is none, you can say something like "I'm ready to listen to new information about X at lunch today; if there's no new information, I'll be out doing Y" (again, in your own words).
This tool isn't a magic wand, so you can think about if it would be a good fit for your situation and your son's personality. It is effective with some people/topics and not others.
The other thing to try, as simple as it sounds, is leaving.
You can think about if you want to announce it at all -- something like "Hey Son, I have a little time now to listen to you about X, and then in 5 minutes, I'll be leaving for the store". You don't have to stick around to keep validating if it becomes clear to you that it's just an endless rehashing and no real progress is being made. The more you are able to gently yet firmly leave, the more chances he has to learn to regulate himself. Yes, you provide 5 minutes of validation, and after that, you are a human with your own life to live.
Will be interested to hear from you if you think either of these approaches might help.
Cheers;
kells76