Let me tell you, you wont be able to talk him out of it any way. He wont listen and if he tells you he will stop he is lying. You will have to decide now in your life..join in or brake loose. Life is about choices and now you will have to decide... do you mix with this type of people or not. Do you, or dont you?
If I were you I'll warn him and if you catch him, just say good buy and get a better guy who is up to your standards.