Alcoholism loves secrecy. Katchie..honesty is appropriate in the world of alcoholism as well as the non-alcoholic world.
dandylion
Dandylion nailed it. Alcoholism thrives in secrets, lies and half truth. I am a recovering alcoholic myself and the only reason it is so is because I was able to look at myself with brutal honesty.
The fact that he is trying to gaslight you and is making up that huge lie for his mom shows that he is not ready to quit yet.
I don't feel he is trying to protect his mom as much as he is trying to protect his possible future drinking! Keep the secret going. Don't get sucked into his craziness.