Hidden, I'm quite shocked that they are going down the personality disorder route at such a young age and wonder if it's down to the family history and them not knowing what else to do.
I'm not a professional, but my Mum is a clinical psych and we have talked in the past about how children are constantly developing and changing from childhood through the teenager and young adult years and that's why it's impossible to identify PDs until they're quite a bit older.
As NorthernLurker said on the other thread, put the label out of your mind if you possibly can for now and focus of getting support to do as much as you can to work on what he needs to help him learn the skills he's going to need to overcome whatever it is that's at the root of his behaviour.
If it helps, my dsis's ex partner had NPD and their eldest is very similar in many ways. Her childhood and teen years were very hard on the family and she herself really struggled for quite a while, but she is now in her mid-twenties and a devoted mum to a lovely little boy, who is the centre of her world and nothing comes before him - so you see, things can and do change. She has had to learn some tough lessons along the way and I do think her df's genes had a part to play in her personality type, iyswim, but that isn't all of who she is and when push came to shove she decided to learn the lessons, take the knocks and make the right choices with regard to how she behaves and treats other people.
She still has tendencies, she has absolutely no appreciation for anything that the family, or anyone else, does for her (and they do a lot) and she is still very much, me, me, me, but perhaps only a bit worse than a lot of people these days. A few years ago everyone was wondering how long we could all carry on supporting her and getting a kick in the teeth back every time, we were worried she would never find her path, but she has and it shows it can be done.
You sound like you are blaming yourself, please be kinder to yourself, it's no-one's fault, it really isn't.
I know you don't believe it, but you are a fantastic parent to have kept on fighting and looking for answers for your ds for all this time.