Marriage changes NOTHING. Please don't go into it thinking that things will get better or that he will change. He will only change if and when he is ready.
Please, please, please don't go through with a wedding....it will turn out to be an extremely expensive mistake if your fiance is still actively using. You don't want to be legally or financially tied to an active addict. Do you want to be dealing with this in two, five, ten, thirty years from now? Do you want to potentially bring innocent children into a situation like this?
If your family and friends know what's going on with him, they will (or should) encourage you to not go through with it. Consider any deposits that you have made on the wedding arrangements payment towards your future inner peace. The money that you've spent for this now pales in comparison to the heartache that you will most definitely live through if you decide to marry this guy.
Good luck and stay strong.