Ask him this question..."The WTBTS tells us to encourage our children to be open about what's in their heart. At what age does this stop?"
He will fuddle. Then go on about how when you are little, and the questions are simple, like "Is there really a devil?" then there's no problem, but when you are older, and the questions become more complicated, you are finally told to just accept whatever you are told no matter how self contradictory.
Say "Why is that Daddy?"
If he's open to it and wants an example of your more grown-up questions, you could ask "If the Watchtower is "the truth", then why do they have to lie to make you believe?"
Then you can pull out the big guns. Start with the lie about how the "early christians" (AKA Russell) supposedly looked forward to 1914 for the beginning of Christ's parousea. That's what the WT says. Many quotes. Yet there is no truth to it. Tell him he will not find even one reference to suggest that, but instead, Russells books are filled with 1874 parousea teachings and 1914 for the end of the world.
So Daddy, why does "the Truth" have to lie?