Oh, one other thing I forgot to mention - if he challenges you for evidence when you tell him something, give him lots of praise. Teach him that this is not something that's socially unacceptable to do, and should be celebrated. Especially if he catches you saying something that isn't supported by evidence - thank him for helping you to learn something.
When he gets different treatment from your father, he'll be confused and won't like it. It'll make for a stark contrast that will sour him on the cult.