You are being given an opportunity to save yourself a lot of pain and sorrow. Please take this opportunity to learn all you can (as quickly as you can) what is ahead for you as the wife of an addict. It's no picnic.
There may come a day when your fiance will be clean and sober--but that certainly wont come any time soon and certainly not by the time you are married in a month. His saying he's not using and his actually not using are two different things. Don't fall for his lies. He's already shown you what he is capable of. It will get worse before it gets better--that is if it gets better at all.