Well for starters, it sounds like you are carrying far more of this relationship than is fair. Personally I wouldn't be calling this BF my "fiancé" because if he is cheating on you, not "manning up" and taking care of his son, or not sharing in the bills ... I wouldn't want to be marrying him and I'd be inclined to ask him to go. Relationships rarely improve once you get married so figure he is on his Best Behavior (not!)
The book, Co-dependent No More, by Melody Beattie is a fast read and has a wealth of knowledge. It will open your eyes about boundaries and such. I would find it offensive if my BF drank to the extent you report yours does.
Once you find your bearings in what an equitable healthy relationship looks like, you will start to feel your power. Right now it doesn't sound like you have any and that you are being used.