Hey AKK, welcome to the group --
It's scary when loved ones have psychotic episodes. Was that his first one?
Glad he got some help, yet here you are watching as he seems to be planning an unhealthy move, only weeks after discharge. How long has he been together with his GF?
Yes, the BPD/NPD pairing is... something else

Does he see you in a positive light these days? Or are you "the bad guy"?
Briefly, re: your question...
I think what we wish for with the pwPDs (people with personality disorders) in our lives, is that if SOMEONE just told them the truth, they'd appreciate it, see the light, and start to change their ways. I'm thinking you'd love it if you could tell your son, "Hey bud, you know all the issues you've been dealing with? Well, your GF is mentally unwell, and it's impacting your sense of self. In fact, she meets many criteria for NPD. It'd be best if you break up with her -- she's not a healthy person." And then, in an ideal world, he'd say, "You know, I never thought about it that way, but you're right -- all the pieces are falling in to place. I'll mail her a breakup letter and cut all ties."
That's not a bad thing to want for your child -- for him to see the light and be healthy.
The struggle is, if he's dealing with PD traits, is that he may be hypersensitive to any criticism, real or perceived; have some ansognosia (denial that there is even a problem), use black-or-white (extreme) thinking, and dysregulate emotionally.
Sometimes, when we share how we see things, and it's an emotionally charged issue (like a possible diagnosis for a romantic partner), it escalates the situation and the pwPD doubles down on their behavior, and can't hear or take in what we're saying.
What do you think so far?
Again, welcome, and I'm sorry you're dealing with so much.
kells76