Autumn - Your fiance is not becoming an alcoholic, he's been one ever since he started drinking.
Alcoholism is a genetic disease. Basically there are those who process alcohol normally and those who don't. Someone with the genetics for alcoholism cannot process alcohol like a normal person. When they first start drinking their whole body goes into a sort of chaos trying to adapt to this poison that their body was never meant to handle. What results is a body that is out of balance and dependent on alcohol. The whole body changes - from the major organs (brain, liver) down to the cells (mitochondria, cell walls). What I have tried to explain is an over simplification of a very complicated disease.
Your fiance is very sick. He needs medical help in the form of rehab. He needs to be seen by doctors who specialize in alcohol addiction. No amount of pleading, begging, or threatening will get him sober. Would you plead with a diabetic to stop being diabetic? Of course not. You would get them medical help, which is what your fiance needs.
Some will tell you to leave him, which is understandable. The painful reality is, is that alcoholism is a progressive disease and many die from it. No one wants to see someone die from a disease that is so ugly and can so distort someone's personality. It is traumatic. Some people never recover from the trauma of seeing someone die from addiction. Many people on here want to save you that kind of pain.
But ultimately, the decision is up to you.
If you want to read more about alcoholism and understand it, I would recommend the book Under the Influence by Dr. Milam. If you ever decide to get him into rehab, I would recommend the book Love First by Jeff Jay.
Best of wishes. You face a difficult decision which will define your life and who you become. If you are a spiritual person, I would pray for guidance.
Panther