When something funny was going on with my ex, my inclination was to fully understand the situation, so I'd start there. Start by getting rid of the rule that has him drinking in secret. You are adults - its OK (under normal circumstances) to have beer in the fridge and liquor in a cabinet. Even in the Southern Baptist church I grew up in, drunkenness was the sin, not drinking. However, my thinking is to get rid of the rule not because it is OK, but because you are no longer in agreement and because it allows you to better monitor the situation. How much alcohol is he going through? Is it every day? When is he drinking? Do you see evidence that he is still sneaking drinks?
Next, I'd follow the money. Alcohol isn't cheap, and neither are prescription drugs, illegal drugs, etc. If you share accounts, go through them and figure out how much money is unaccounted for. How much cash is he spending each month? These days, most people, especially young people like you guys, don't tend to use a lot of cash. Check your credit reports for unknown credit cards.
Finally, make him get a full examination. Talk with the doctor frankly about amounts of alcohol drunk vs. the body's reaction. It is rare, but his body could be metabolizing alcohol wrong, it could be interacting with another medication, or as other have said, he could be drinking way more than it looks like or mixing it with something illegally obtained. He also could have a mental issue that is coming up now, and this is a symptom of that. I had a person in my life who turned out to be bipolar, and while that does not excuse their bad behavior, it helps me understand that they were chasing new exciting things to artificially create a "manic" state during a "depressed" state. I might have approached things differently armed with that information up front.
These things at least give you the tools to understand what you are dealing with and make a good decision. We can all arm chair quarterback all day long, but you are the one who needs to turn this into a situation you can live with, either by getting to the root of the problem and fixing it or by getting rid of the problem (which can never really be done in a situation with a child).