Sarah, if you were to know the ins and outs of my life story you would understand were i am coming from.
To be brutally honest, he is not the person you may have fell in love with anymore. If he is a addict, he will be getting further and further away from his real self every day. You will cease to be any use to him in the real world, you will be a victim of his addiction, merely there to facilitate his habit.
I wish i could say that you should stay and support him, but as cold as it may appear i truly believe you will be wasting your life hanging on. You may say how could i assume he is that type of person, it is because it is out of his hands now. That is the nature of the drug. His only chance was to make the decision not to start in the first place before the addiction was in charge,he has lost that option, he is no longer in control.
It really is not his fault anymore, no one would engineer the lifestyle that he will have to endure, it is thrust upon them by the drugs.
There may be a day when it all turns around for him, i truly hope there is.