How much should he be drinking?

You show some advanced maturity for someone your age if you're asking this question. Your concern for his well-being is understandable, but unfortunately, so is his behavior. There were a few nights after I turned 21 that I drank more than I should have, although it wasn't habitual. If his grades do start slipping and he starts forgetting important things, or becomes unreliable, that's when you'll need to say something to him. I wouldn't justify his partying lifestyle at all.

Remember the movie Super Size Me? Morgan Spurlock, the director, also did an experiment where he had women consume the amount of alcohol that most 20-somethings imbibe over a period of 30 days, and the results were pretty much what you would expect. Google "Morgan spurlock alcohol 30 days" and see what you come up with. That should give you an idea of what the level of alcohol is consuming might do to him if he keeps it up.