Ok, the paranoia is unfortunately a known symptom, so it's real for him, too. I'm sorry he lost his son, that's awful. I would like to very much recommend that you read the book "Stop caretaking the Borderline or narcissist, and how to end the drama." I have to tell you, that he is apologetic is huge, and that he in general is even willing to do therapy is huge, too. That he listens to you is very good, too. He clearly wants to get better. Meditation is very effective for BPD, he should try that, very much! I'd say take care of yourself, read about it, understand the pattern and trigger, and make him going to therapy (DBT seems to be the most effective, if he can't get counseling for it there are books on Amazon that seem to be very effective) a condition for being with you. He seems pretty unwell, but the willingness to work on himself might get him far. Be well and safe.