It sounds like your boss suspects you are drinking and it's just a matter of time before he confronts you. And if he is savvy, he will confront you someday when you come back from a liquid lunch. And if you are subject to a breathalyzer test, well, you're screwed.
It seems to me that you have 3 choices.
Stop drinking and let him always wonder.
Keep drinking at lunch and lose your job.
Or talk to your boss, tell him you have a problem and that you want to seek treatment for it.