Do you normally tell him everything you do and every place you go?
You can always just say you're meeting some friends for conversation (true). Or tell him that you are going because his drinking bothers you. It isn't about him, it's about you. He can be as "against" it as he wants to be. This can be a good place to start working on boundaries. He does what he wants to, so far as his drinking goes, and you do what you want to do, so far as his drinking affects your life.