Hi Corkie
Sometimes when we set a boundary and make it known, it sets of a chain of unexpected consequences. If the consequence is a big one, yet the other person wants to continue the action bad enough (and they are a little slippery in character), then it makes a warped kind of sense to hide and lie. Because they want to have their cake and eat it.
If I were in your position, I wouldn't mention it again. Just sit back and smile and observe. Because if he is drinking again, you will know. Without a shadow of a doubt. I get the sense that your senses are highly tuned. But he won't tell you, because he has too much to lose. So, if you are serious about leaving if he drinks, you don't need an admission from him, nor his permission.
Trust your gut. And don't rely on him to confirm it.