Can you not block his number completely?
He is a controlling bastard - BPD or not, he has got issues that you do NOT need to get involved with.
I had a relationship with someone that had serious issues - I forgave him so much because I knew his family background so had sympathy with him and thought I could understand where it was coming from. So maybe I could understand it - but in the meantime he was turning into a nasty controlling abuser, and I was letting him because I sympathised. Sympathy is wasted, tbh - other people have nasty backgrounds and don't become abusers. Other people have disappointments in life and don't become abusers. People CHOOSE to become abusers - this bastard knows you've put up with shit before from the narc you had DC with, and he's testing to see how far you'll let him go, in the meantime despising you for allowing him to go that far.
Whether or not he's a classic abuser, he is definitely a BULLY and you do NOT want to be with him. His definition of love is a load of shite - it's more about ownership of you. He wanted you, to himself - you went off and besmirched yourself with someone else, now his precious possession (you) is secondhand and he hates you for it, while still loving owning it.
As someone else said further upthread - he is never going to resolve this conflict because his "love" for you is not healthy and isn't love - it's obsession with ownership.
Please keep trying to get rid of him - do NOT talk to him, except one last time to tell him that if he persists in attempting to contact you, you will report him to the police for harassment. You must warn him that you will do that, and then cut all contact. Your DC do not need this person in their lives, nor do you.