We are no more responsible for when a person uses, then when a person stays clean. We do not have that kind of power over people. We can only control ourselves. If an addict isn't staying clean for themselves and because they want to, it's doubtful they will stay clean very long.
Maybe you can stop talking to him. Maybe you smile, nod and just walk away. Everytime he opens his mouth just envision cartoon bubbles coming out saying "blah blah blah" or "quack quack quack" and then focus on how you are going to improve your situation.
I read you just broke up. What are your plans as far as future residence?