Boy is that ever a big question. "Is it a disease?" Addiction is surely a physical condition... whether or not you choose to refer to it as a disease seems to me to be largely a question of semantics.
Something made him turn to chemicals in the first place. It could be something as seemingly small as boredom. I urge you to read some of the posts on the NA and naranon pages here. I think you'll get a lot of info., and one of the ideas you'll see repeated an overwhelming number of times is that an addict requires a LIFE CHANGE if he is ever to be free of the urge to injest mind altering chemicals. Just detoxing is rarely enough. Now, it follows, that if a life change is required, there was probably something wrong with it in the first place.
Along with is there a void to be filled and is it "just" a disease, there is also "is he just a jerk". (Oooh, hit your protective button, didn't I?) But even if one cares to descend to labels of that sort, don't you kind of think that someone who is so self absorbed has some kind of a problem?
Go to the al-anon home page (http://www.al-anon.alateen.org/) and read some of the information there. You are wise to want to know as much as you can. But I'm afraid you may have to refocus from helping him to helping yourself. Nothing you are doing makes him drink, and there is probably nothing you can do to make him stop. He has to find that motivation himself.
Smoke