I would say something like: "You can stay until October as long as you pay the rent on time. Pay the back rent you owe within the next xx days, and you can stay. Otherwise, I will have to ask you to leave." Keep it neutral. If he tries to lay on a guilt trip (which he most likely will as addicts are very manipulative), say: "I love you. You are smart. I know you can handle it." Leave it to that.