Well Blackrose I will wade in with my 2 cents. No one has to agree.
NOT YOUR FAULT. NOT your fault your Ex Relapsed. NOT your fault this kids Father is who he is. NOT YOUR FAULT.
Every action has a consequence. He has decided you do not deserve respect? Fine. He is 18. The door is right there.. don't let it hit him in tthe @$$ on the way out.
Is that tough? YES. Probably harder on you than on him.
If he asks what he can do so that doesn't happen? Tell him NOTHING. He is 18. He needs a job and a place to live. He best figure out both and, BTW, he will find out he had a good deal with you. Tell him you love him but you cannot live with him and he cannot live with you anymore.
THEN (the hard part) STICK TO IT NO MATTER WHAT.
Love does not meaning putting up with abuse. Even a dog.. loyal as they are.. will not come near its owner if it is abused often and badly.