....I'm not sure what I'm looking for by posting here... I guess just getting even this very abbreviated version of events out here is a relief. Any advice at all would be so greatly appreciated.
I went through a lot of this, and for the most part things have been okay, or at least not as bad as I feared, in terms of my daughter picking up bad habits from BPDxw.
Get a therapist for your son, maybe, and have it written in the divorce decree so that your exH can't take your son to different ones until he finds one he can manipulate and control. And given your son's age, it might be prudent to have succession rules written in the decree to choose a new therapist if the current one moves or retires.
Beyond that, just be a good parent and role model for him. The advice my D's therapist gave me was to help nurture their mind and thought processes so they learn to think for themself. If your ExH is filling your kid's head with nonsense, don't panic, just provide some guidance to them. Ask them what they think. If they ask questions like "[Parent] said this, is that true?" Ask them why they want to know. Ask them what they think. And then tell them what you believe. If they feel conflicted, tell them it's okay to be confused, and adults don't always agree, but [your child] can make up their own mind.
I was always careful not to criticize my BPDxw in a way that would hurt my daughter, even if my daughter was upset with her. I recognized that she's still half her mom, and so I don't run her mom down. I may say things like "your mom has her own opinions and I don't agree with them" or "It was wrong of your mom to say that to you. You should tell her how you feel next time."
There's nothing you can do to make the BPD parent behave; you can just be a source of truth, patience and peacefulness for your child. Think long term here, and don't sweat the small stuff.