Alcoholic - intervention needed

I've been friends with this guy for over a year and over the past few months(maybe 6) we became more than friends. He's been struggling for a number of reasons and has slipped into alcoholism. I've mentioned it once or twice but he brushes it off and shuts down. Now it's starting to get really bad and I'm afraid for what will happen to him. I feel like I don't know what to say to get him to take me seriously and to understand how serious I see it is for him.