For this you need to drink 10 drinks in a single game. This includes beer, whiskey, cocktails. You can buy drinks as well. Once you drink 10 you will unlock him. I did this in coop and had my partners drop all alcohol for me. You could also store all alcohol in the atm, and then once there's 10 take them out and drink. Just remember it costs money to retrieve items from atm. Once again the easiest is to have your friends drop all alcohol for you.