Just throwing this out there to see if it might be of any benefit to you at some point in time.
When he said "I believe that the GB are the only true word of god on the earth"
Carl Sagan said "I don't want to believe, I want to know"
You could say that "believing" is not the same as "knowing" and that alot of people "believe" in something that is not true.
Lets say that you are buying a house, putting your entire savings into this house and you are told by the agent that they "believe" that the termite treatment was done. You would not be out of place to ask to see evidence of the treatment and that belief alone is not enough to satisfy you since buying a house is such an important investment. What if the agent that was trying to sell you this house said that you should not be asking to see the termite report and that this was the only house for sale and that if you keep asking to see that report that they will never sell you the house.
You would back away from that person as you would think that more than likely their is a reason that they do not want you to keep asking about the report and the obvious reason is that it does not exist.
Then I would mention that the only indication that a person can logically use to see if the GB is actually gods mouthpiece would be to see their track history in what they said would happen, and what actually happened. Looking ahead to the future promises are not valid since, of course, they have not happened yet.
Tell him that you will only use WTBTS published materials to use to see what they said would happen vs what actually happened.
Then show him the failed prophecies about 1874 1914, 1915, 1925, etc
Then explain how a person should not be biased into believing something based on the consequences of that belief. I think that many times we make decisions as for as what to believe based on the desired percieved consequences of what that belief would deliver.
Argument from Consequences
Arguing from consequences is speaking for or against the validity of a proposition by appealing to the consequences of accepting or rejecting it. Just because a proposition leads to some unfavorable result does not mean that it is false. Similarly, just because a proposition has good consequences does not all of a sudden make it true. As David Hackett Fischer puts it, “it does not follow, that a quality which attaches to an effect is transferable to the cause.”
In the case of good consequences, an argument may appeal to an audience's hopes, which at times take the form of wishful thinking. In the case of bad consequences, such an argument may instead appeal to an audience's fears. For example, take Dostoevsky's line, “If God does not exist, then everything is permitted.” Discussions of objective morality aside, the appeal to the apparent grim consequences of a purely materialistic world says nothing about whether or not the antecedent is true.
One should keep in mind that such arguments are fallacious only when they deal with propositions with objective truth values, and not when they deal with decisions or policies [Curtis], such as a politician opposing the raising of taxes for fear that it will adversely impact the lives of constituents, for example.
(taken from https://bookofbadarguments.com/?view=allpages )
NJY