Kevin, what a courageous and loving journey you describe (both you and your son). Seeing your (adult) child in pain and not being able to do anything about it is excruciating.
The fact that your son knows that you are there for him is probably most helpful and hopeful thing you can bring to his life. But you sound like you are like me, parenting my adult child, and many parents like us - you need resources.
Firstly, what about you? Do you have things in place to keep yourself together? Then, as far as your son is concerned, sounds like he needs help with emotional regulation. That's good news. Those are skills people can learn. He probably already has a few things that work but maybe not the awareness of when his body is going into survival mode. It helps to understand the physiology. Then he will know that he's not 'crazy' or 'defective' or any of the other labels people acquire and even give themselves.
Please feel free to private message me if you'd like to continue the conversation. And in the meantime, my hat is off to you: What a beautiful, determinedly loving and caring person you are and how much you must be hurting to see this young man struggle.
Lou