It all depends on your goal.
Is it to convince him that 607 is wrong?
Or is it to make him understand why you think 607 is wrong?
They may seem like the same thing, but they're not.
If it's just to show him why you think it's wrong, then YOU get to lay the ground rules. You can tell him what kind of evidence you'll accept and what kind you won't accept. You can say: "I don't trust the watchtower not to lie, so I would accept their articles as a source."
On the other hand, if your goal is to convince him that 607 is wrong, then you need to follow HIS rules, you need to ask your dad: "What kind of evidence will you accept? Will you assume that every non watchtower source is lying if they disagree with 607? Or will you accept archaeological evidence if it's different from the what the watchtower says?" in other words: "What would it take to convince you that the watchtower is wrong about this?" and if the answer is: "Nothing could possibly convince me" then you're off the hook and can spend the rest of the evening talking about something more fun, like toenail clippings for example.