Love. I truly feel so sorry for your situation. You are in a kind of purgatory.
I've said before and I'll say it again. you need to detach from this dickhead.
involving yourself in his sordid life is hurting you.
DON'T speak to your ExH, don't do the joint shop with him, there IS no joint anything anymore.
Just get your son a special gift from YOU that he can keep as a token of your unending love and admiration of him. Let his father fuck it up. So what.
This is the start of your darling son's adulthood, let his first step as a man be one where his mother shows him how proud SHE is of the man he has now become, and by doing that she shows him how strong SHE is. He can then hope to emulate you and your undoubted and obvious strength.
The job we have to do as parents is raise them through childhood, prepare them and give them the tools and the strength to cross the bridge into adulthood. Once they cross that bridge, they can stand on their own two feet, make their decisions, and know when to seek counsel. We will always be there for them, but they need to live their lives. Your H is incapable of helping his own DS to cross that bridge, so you must do it. You are the one he will come back to, you are the one that has been there for him.
when you take this solo step away from this treacherous twat of an ExH, you will see your own strengths, you will see that you are being held back in life by hanging onto this man who is not worth your time.