Originally Posted by
firebolt 
He hides it because he knows his drinking upsets you, but he doesn't want to and or can't stop.
This. He hides it because it upsets you.
For whatever reason, I am not upset with my ABF's actual drinking (it's the emotional intimacy issues that bother me) so he has always been honest with me about how much and how often he drinks. His relatives have NO idea the volume of booze he puts away. Of course they don't know because they react to the drinking. "Have you been drinking" is the quickest way to get an active alcoholic to lie to you. Checking his soda cans will only inspire him take more precaution in the future.
That you feel it's your responsibility to help him is more than reason enough to get yourself to Al-Anon. It doesn't matter what you sweep under the rug, ask him about, check on him about or let him alone about - HE WILL DRINK. He will drink until he chooses to get himself into recovery - or not.