Drunks have often been nurtured by an over-forgiving, over-indulgent mother/ abusive and negligent father combination. You probably appear an attractive safety net who can have all the stress of day-to-day life while he focuses on getting plastered.
Set boundaries early on in any relationship(romantic or otherwise) that say you're not there to constantly pick up the pieces. You want to be put first, and you should be, but be prepared always, situations can arise every day where a piece of your self-respect is called upon to be sacrificed. That's how it starts, until it's all given and gone; be ready to draw the line and show that you won't tolerate crap. You've got what they want, not the other way around.