I am going to play devil's advocate on this one. IMHO the train needs to slow down on this one. This is probably not going to be what you want to hear, but I think you should at least get one person's perspective that is different.
Leaving him, or threatening to leave him, now doesn't make sense to me. Your man was in it way over his head from the sound of things, yet he was able to kick the OCs and H. You stayed by him then, and he was still able to stop his use. I say "still" because you are worried about being an "enabler", and it sounds like you blame yourself (at least partially) for his problem.
Now he is on subtex, which is troubling to you because he is still abusing them by not taking the medication as prescribed. Is it the fact that he is snorting them that is bothering you so much or the fact that he is buying additional meds on the side? Or is it simply the fact that he is still using anything?
The reason that I ask is because it is very difficult to get any sort of high off of subtex (unless he is IVing them). I can't speak from experience, but even IVing subtex isn't going to be anything compared to H or even OCs. My point is that even though he is snorting them (in large quantities) he should still be able to function - i.e. go to work, help around the house, etc. If he is nodding out all the time then he may still be using H, OCs, or some other full agonist opiate. I am not in his mind, but if I were in his shoes I would feel like a ex-bank robber getting his balls busted for jaywalking.
While he is still technically "abusing" it honestly sounds like he is in a much better place that he was in the past. I am not trying to condone his actions, and not trying to talk you out of being upset about them. What if he was going to a methadone clinic everyday and had to stay on methadone maintenance for the rest of his life? For some people that is the only way that they can live their lives, but they are fully functioning members of society. Would you be able to accept that?
All of that said, if you have done your research on subtex and maintenence in general and you know that you can't live with it then I would try the following. Go to him and ask him in a genuine, sincere voice what you can possibly do to help him stop. Tell him that you will do anything in your power to help him, and mean it. At this point it is likely that he wants to quit, but he is scared of the withdrawls. If he isn't then he should be, because subtex is no joke to kick. Ask him what a realistic timeframe is. You have to transition from the role of someone that he feels is nagging to someone who is a partner on his side trying to help him stop.
Finally, you should tell him no matter what happens you will love him and that you are on his side. There is always hope no matter what.
This is just the 2 cents of an ex-IV H junkie.