I just would like to add that I disagree with those that say you should re-evaluate your relationship with your fiance. There are PLENTY of been married for a while parents that have discipline discrepancies. I don't think it has anything to do with re-thinking your relationship or anything (from going by what you said).
Well, there needs to be a mutual understanding between you two when it comes to discipline. What you were doing all along was working, but you have another adult living there that is your PARTNER (you two are a team). Imagine living with a child everyday and NOT disciplining them at all- that could be quite frustrating! I'm not condoning his ways however, because he should know that it is bothering you and respect you.
Perhaps especially since you two are a fairly new couple, some advice from a couples therapist of some sort (I know a lot of people look at that as scary or a sure sign that your relationship is doomed, but it isn't...). You may just get all the answers you need in the first 2 or so sessions. What a therapist does is help tell the other partner how the other one feels while validating their reasoning. It's very helpful, and you both should feel more understood afterward.
If it isn't something you are interested in, then try having the serious conversations on your own with him. And keep in mind that your opinion isn't the only one that matters because he's YOUR son; instead, try to find a happy medium. Your positive reinforcement may have worked, and may work now, but that doesn't necessarily mean that it is the only way that would work or that there is no room for some changes invovling the way you used to do it. It is never a good thing to walk away from decisions (especially big ones) that were made when one person feels cheated and not heard. It needs to be as close to a win/win as it can get.
hope things start getting better for you,
Rebecca