Your fiancé may sincerely plan to stop drinking once you are married - but almost every week I get at least one letter from someone who found they (or their spouse) couldn't keep this promise. If he won't (or can't) stop now, what reason will he have to stop once you are married?
In other words, the time to confront this problem is now, not later. If you and your fiancé fail to do this, you'll only be opening the door to more serious problems later on. The Bible says, "Wine is a mocker and beer a brawler; whoever is led astray by them is not wise" (Proverbs 20:1). Almost nothing destroys a marriage or a family more quickly than alcohol, and it would be foolish for you to think otherwise.
Urge your fiancé to ask himself why he turns to alcohol. Is he hoping it will help him deal with stress or pressure? Is he trying to escape from some problem or fear? Is he simply giving in to the pressures of the crowd? Whatever the reason, he needs to face it and deal with it. If he needs help, your pastor or doctor will know what resources are available in your community.
The most important thing you can do, however, is commit your life - and your future - to Jesus Christ. God loves you, and He knows what is best for you. Put Christ at the center of your life, and seek His will for your future - including your marriage.