A case a day is a LOT of beer, especially at 20. Perhaps more telling, though, is his reaction when the subject is raised. However, other than telling him that you are concerned and how it may impact his life, these are not your choices or consequences. I was drinking in a similar fashion minus the great grades in college at his age, despite the fact that my mother was an alcoholic. I refused to acknowledge the connection between my drinking and the possible outcome (which I abhorred). It took me another 20 years to surrender, during which my mother got sicker and ultimately had to be removed from life support. I kept drinking for another decade after that.
The sad truth is this: You cannot make someone else "get it." He will have to learn this in his own way on his own timeframe.