My personal bias, is simple well thought out questions.
Example: "Dad, although I am not a believer, I have wondered why the Watchtower teaches 'publishers of the Good News' to avoid and hide the "Good News" according to Paul. Something does not add up since more than half the Bible's 152-odd references to 'Good News' are by Paul."
The strategy here is to point your Dad to exactly the mechanism which by design has liberated millions (like Ray Franz) from mindless addiction to religion.