"the part I still haven't been able to understand is why so many have told me to completely disassociate with him entirely."
It is very unlikely that this person is going to settle for a non-toxic normal relationship with you. It is very unlikely that he will accept your "help". It is more likely that he will drag you down with him. You are not dealing with a normal person who will benefit from a little kindness and friendship. What you have described is someone who is deep in their addiction, despite copious negative consequences. And, despite already having people (his parents) and resources (detox and rehab), he has continued on his destructive path.
No one is calling him worthless garbage, scum of the earth. The point being made is he has to be the one to choose change. You don't get to do that for him.
There is an expression around here - "let go or be dragged". Believe these people who have been in the trenches with addiction. Your addict will not care how far down that he drags you.
