It's nice you were able to talk to your Dad about it. It's interesting to see different people's reasonings for staying in. I recently talked to my Dad, and he basically felt that the organization makes mistakes...and even admitted they cover over their mistakes which looks bad!!! But, it's "the truth." No other group preaches to the extent they do, and the world is a bad place that has to be ending soon. He also mentioned he has no regrets about being a witness, but I bet he does because the religion is messing up my life and it messed up his relationship with my sister.
My Mom told me that she feels that there is one God, but he's not going to punish people who love him, no matter if they go to the meetings or not. She's tired of religion!
To attempt and plant a seed of doubt, I said most of their friends have left "the truth," even the guy who brought them into it and his parents...they must have a good reason. And an elder also left from our hall, why? He also must have had a reason too. I told him it was the UN issue that bothered his conscience.
There isn't reasoning with them, only seed planting. My husband realizes that the organization makes mistakes and had some pretty crazy and even stupid ideas published, but "it's the most right" religion to him and nothing is going to change that.
Maybe your Dad could help remind you why it's God's organization, but the WT says we're not supposed to ask that question and it could hinder further conversations with your Dad.