mayer,
You have said that your parents will be helping pay for the wedding. Have you considered telling your parents what you have learned about your fiance?
Others here may disagree, on the view that you are an adult and your life is not your parents' business. But my view is that (1) accepting money from your parents for this wedding makes your wedding at least partly their business; (2) if you decide to cancel or at least postpone the wedding, you will have their understanding and support in taking steps to unwind the plans;
(3) if you decide to marry this man in spite of your well-founded misgivings and all the red flags from everyone here, you may need to rely on your parents for help (whether emotional or financial or both) further down the road. Your parents deserve full disclosure about what *they* may be getting into, as well as what *you're* getting into. (4) Keeping your fiance's addiction a secret from your parents uses emotional energy that could be better channeled into deciding the best course for you.