Hello Bestfootforward and welcome to the BPD Family forum! I am very glad that you are here, but also very sorry for the circumstances that bring you. You are in the right place to find oodles of support, empathy and understanding. All of us here have walked a similar painful path with our own children, so please know you are not alone.
The best place to start learning is to peruse the resources and lessons on the right hand side of this page

You will find excellent information on how to best communicate with your son and how to change your approach if what you are doing now is not working. Also, reading the experiences, stories and advice of others' posts on this site is enlightening and some of the best therapy you will find. I know I personally found the validation of other parents to be very liberating in allowing me to move past self-doubt and guilt and start working on changing my approach with my BPD dd21.
Again, a warm welcome to you, Bestfootforward. Please keep posting, share as much as you want, as often as you want - it really does help! Looking forward to getting to know you, MomMae