Can you describe what makes you think he's an alcoholic? Your guy is likely right but there are people who abuse alcohol who are not alcoholics.
I would highly recommend resolving this before marriage is considered and uhm... Use reliable birth control :-)
I would also suggest counseling before marriage. Most marriage counselors say people get to them 2-3 years too late. If he is not willing then that would be a deal breaker for me because I'd someone won't agree to that before the wedding they won't after and alcoholic or not, marriage is hard work. I'd rather have someone with more flaws and more willingness to change ...
There are signs and stages of alcoholism, do some googling and see what warning signs there are but trust your gut - in a marriage you don't need a majority vote to declare a problem, one vote is enough.