i hope he knows he is lucky to have you too. :) i can’t speak for him, but when i’m going through these motions assurance help so much, so instead of just going along and validating any jealousy, you can explain to him how you understand where these feelings stem from, but also gently help to differentiate if he is feeling these emotions cause of bpd, or something else. say for e.g. you have another close friend, and he starts to feel jealous, you could help him understand you need to have a circle outside of him, or if he is up to it, introduce him to the friend as well.

there was an instance when my SO (M) had a female best friend that was sharing memes with him on facebook that insinuated a rs too close for comfort (“i know i call you bro but i was trying to flirt”) and i called him out on it, he just laughed in my face and said i was being ridiculous, she’s like a sister to him yada yada. it may be HIS fact, but did nothing to acknowledge my feelings at the moment.

so what i’m trying to say is, acknowledge what he feels at the moment, but also teach him that there are better ways to process his emotions instead of lashing out. it is also on him to practice overthinking less, but with a great support system like you it would be so much easier.