so you are involved with two of your exs?? what does the one ex think about you being involved with the other, and going so far as to pay his BILLS? when you know that THIS is why he is broke?
Truth is, he lost all of his money on this trip and owes money to a dealer. This trip came 1 week after losing his job because he stopped showing up on time. His quick fix solution is to get back in the game and deal enough to cover his bills. I see that as the beginning of another end. I even told him I would pay his rent if just walks down the street and gets a crap job at McDonald's or something. He's not hearing it and determined that dealing is his quick ticket out of debt.
i suggest this isn't about you wanting to stick around and help him get clean, cuz he's is anything BUT clean. but you have nominated yourself to be his savior...the ONLY one. and that's a set up for you AND for him. as long as you keep the door open to "help" him he'll keep showing up, looking all pathetic and needy.
you've got two men on the hook, you need to pick one. or pick none. and be really honest about your own motivations. if he wants real CLEAN AND SOBER role models he can go to AA and/or NA and learn from the experts who've been there and done that. he has a lot of options, make sure you aren't trying to limit those options to just YOU.