Why one becomes an addict and another doesn't still is the 64,000 question. My mother is an alcoholic has been for 65 years, my childhood was pure hell, however, I am not an addict or alcoholic, why? I have no clue, all I can say is that I made the choice to not follow in her footsteps. I had to forge my own way, with no help, no enabling, no nothing. I did it, and, it wasn't easy.
My point is that I did not expect anyone to save me, to make me right, I knew from early on that it was totally up to me.
Maybe its time for him to stand up and be counted, to finally be given the chance to find his own way. You are cheating him of his divine right, to be a responsible adult. He can take care of himself, if that is what he wants to do, however, if you keep changing his diapers it will never happen.
I know you love him, sit back and think this through, what has gotten better with all your enabling, my guess would be...nothing....and,as nothing in life remains constant, it either gets better or it gets worse.
All said with caring concern.