There are lot of good questions that you can ask your father that might get him to do some serious thinking. For example:
We are in the most crucial point in human history, would it be logical for God to allow an organization representing him to make so many mistakes during this time of the last days?
Would a religion who really served as God's communication channel blatantly disregard the bible and run ahead of God time after time proclaiming dates for armageddon? (Jesus warned us on several occasions to watch out for individuals claiming that the end is here)
Also a true mark of God's chosen people would be one who cares for the flock. Is the WTS really any different than other religions in regards to child abuse? (have they always turned in known child abusers to the local authorities, have they removed and kept these abusers from positions of authority and trust)
If your dad really thinks about the answers to that question then the truth should hopefully be evident