I'd revisit the idea of separation. Maybe consult a lawyer about your options if he won't voluntarily agree to one.
And yes, the secrecy and lying just go with the territory. He's not doing it to hurt you, he's doing it because he wants to drink and knows you don't approve. So clearly, you don't have to TELL him again that you don't approve--he gets that part.
He is spiraling down even though you are there. His claim that he will spiral if you leave is just his wanting things his own way--you there and for him to be able to keep drinking.
EVERY alcoholic wants desperately to be able to drink socially. The fact is, they can't.
Have you been to Al-Anon? It might help you get your head on straight so you can make the choices you need to make. There is no emergency here--at least none that you've mentioned. So you can take your time to decide how to proceed. Just know that your choices won't dictate his. Your leaving may or may not be a wakeup call for him. Many alcoholics, even though they love their partners, are unable or unready to quit drinking. Some have to accumulate many, many losses before they are ready to stop. And some are NEVER ready to stop. They just keep drinking until they die or something else happens (jail, hospitalization).