Of course it is possible that he has "seen the light". A heart attack is a pretty serious eye-opener. It would be wonderful if he woke up to all that he was damaging by his drug use. I would do the same thing you are doing, although I have to be honest. My daughter has been on life support twice (that I know of) and has had multiple hospital admissions because of her drug and alcohol abuse and their consequences. I have sat with her in the ER and visited her when she was admitted. I have never been able to stay away when she was in serious medical trouble, because I never knew if she would survive. That being said, if I were you, and he starts the addiction pattern again, even for a moment, I would suspend the relationship and insist on treatment. That is where I am with my daughter right now. She says she is waiting to get into rehab, so I will wait to see if she follows through. And then I will wait to see if she gets into an aftercare program. I hold on to the hope that one of these times she will be serious about getting better. Your fiance might just have been scared enough to realize what he was throwing away. Just don't let your guard down. Don't ignore the signs.
Hugs and prayers!