I also divorced my abusive husband... and that was at the time of the birth of our son... so it was a long 18 years.. and I am the one that slept with the knife under my pillow.. And my husband did rape me...I have PTSD...and my ex-husband for many years lived close by.. or sat in a car outside my home...
I'm sorry.. I don't know what NPD is...

So to say I understand.. I do....

Coping skills... alarm system for night to sleep better.. solid locks, solid doors.. last one in bed..so you are the one to check everything.. two telephone numbers.. one for your ex... depends on whether you are required by law to give him one..window locks.. special patio door lock..I kept 2 70-75lb big dogs.. with noisy barks.. learned self-defense protective moves. garage door opener.. driving in.. and keeping doors locked until garage door is down.. watching to insure no one sneaks in while it is closing..
All of these measures helped me... feel more in control.. of my situation...
Went to the police department.. attended some group support counseling meetings of men that abuse their wives/partners... manatory for them as a part of their patrol.. to be in their midst.. so that I could conquer my fear.. to get angry with them... to ask them why..
it was empowering to me...they couldn't do anything...

Shock came when after our divorce, I found out that he had done to his first wife.. and I got scared even more..

And relief came for me.. when my ex was forced to move after many years.. due to many factors that I won't mention..

If he hadn't moved.. I would have after my son turned 18.. as time went on he found new targets for his anger.. so thru the years.. that part got better.. but never went away..
until his physical health got bad..

I wish I could say "you have nothing to worry about" so you are comforted.. but to do that would be a dis-service to you..
It is good to keep your guard up.. and I have the greatest compassion for both you and your son..

((((hugs to you and your son)))))\