can someone change? Advice..

I've been with my boyfriend for a year and a half. Both of us have said we wanted to get married and have kids, he already has twin girls. Well.. he has an issue with drinking, and when he drinks his whole demeanor changes. It makes me upset and causes me to get an attitude and mouthy. We sat down just last week, not the first time either, to talk about how I don't like being around when when he starts drinking. He told me it would slow down. Well yesterday he was drinking while I was at work and when I came home I could tell he'd been drinking. He flipped out over the littlest thing and it caused a huge fight... I ultimately left. He is blaming me for all of this today. I love him and his girls and I love who he is when he's not drinking. Can someone really change? Am I wasting my time? I hate to feel like I'm being cheated on with alcohol, or that Mr. Budweiser is more important than this relationship.