he is very much held in check by "but who else is preaching about the kingdom of God world-wide like Jesus said"?
I don't think you should give him an answer. Nobody compares to JW's, who would want to?
Turn it back on him to prove that JW's are preaching the Kingdom of God. Where in the Bible does it describe the activities peculiar to Jehovah's Witnesses? Ask him to read the book of Acts in context and see if the preaching of Jesus' apostles and disciples in the first century compares with the activities of JW's. The book, '"Bearing Thorough Witness" About God's Kingdom', currently studied at the "Congregation (cough) Bible (cough) Study" cherry picks scriptures, takes them out of context, twists them, and extrapolates them to fit the Watch Tower brand of exclusionary Christianity.