Hummingbird,
I would be very concerned too. That's a lot of beer. Perhaps with the lesser alcohol content, he is able to function, drinking pretty much two beers an hour, if he's up for 12 hours.
I do know that arguing over it will only probably make him feel entitled to drink more... that seems to happen if I argue with my son about his drinking. There isn't much you can do, unless you make him leave your home. Letting him know how you feel isn't wrong, said calmly and with love, maybe when he isn't drinking.
I am sorry , as I know how it breaks our hearts when our children, grown or not, are abusing themselves in some way.
I wish you and him the best. Al-anon is great, at helping us work through the pain, to healing and peace about what our part is in the whole thing.
hugs
chicory