Gentle reminders, such as to 'never go beyond what is written' in the Bible (1Cor 4:6), may be helpful, since JWs do it on almost every subject.
The same idea is derived from 2Ti 3:16, "All scripture is inspired of God, and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness..." Witnesses hear this scripture all the time, but rarely apply it.
Much more emphasis is given to what the Governing Body says than the Bible, and the Governing Body itself is a concept that 'goes beyond what is written', since it is not found in the Scriptures.
Even though he will have to stick to the prescribed outline in giving the talk, you might be able to make inroads in getting him to think, and to see that Witness beliefs and concepts are sometimes not based on the Bible at all.