Perhaps ask him what evidence there is that he has the one true religion. The witnesses believe that they were chosen by Jesus in 1919 but what happened in 1919 to support this? What proof is there? How do they know that the 8 bozos in New York speak for God - what proof is there?
Fact is that they do not believe what they were teaching back in 1919 when they were chosen so either it wasn't truth then or it isn't truth now.
I asked my own family this question as they want me to return to the meetings. I said if they could prove to me that the Governing Body is who they say they are then I would return. Their response was "they do not owe you proof". There simply is no answer to this question because it is not the truth.