"New Light" is a joke. It is an excuse to cover up bad reasoning and policy. Ask your father how many times since the bible was written has it changed it's original message. When he says none, then ask him how many times the WTBS has changed policy. If he were to say that he believes the same things as Rutherford, he would be D'fed for apostasy. There are numerous examples on JWD to show policy changes. After quoting several, ask him if the FDS claims to be "inspired" by God to write what they do. When he responds in the affirmative, ask him why God changes his mind so much.
There is no other way to explain "New Light". If the WTBS is "Inspired" from God, then God must have made some obvious doctrinal mistakes. He will then tell you that the Society is made up of imperfect men. This is true and has always been known. That is why they cannot have it both ways. They are either inspired and God's mouthpiece, or they are imperfect men who don't really know anything other than what they can guess at. If they are inspired, then God has made many mistakes. It cannot get any simpler than that.
Don't expect a reasonable outcome when you say this. I tried it on my dad and was thrown out of his house for being an "Apostate". The funny thing is, I only spoke about things written in their publications.
CCS