Before your meeting buy some index cards and write certain JW phrases on them that you think he will say when you questions his
involvment in a CULT.
Then when he uses those phrases, just pull them out and show him that you knew that he would give that answer. Also possibly state that he might feel light headed and or angry even though you are 2 adults discussing an adult topic and how this is another "click" to show that he might be in a cult.
I personally would use the phrase "High Control Group" initially. The thought I'm trying to share is to write down things ahead of time that you think he would say, to show him that "this is what members of HCG say" when they are asked if they are members of a HCG.
I would also educate myself ahead of time with Steven Hassans definition of a Cult or HCG to show that it is not "specific beliefs" that determine of a person is in a HCG, since there are SO many varibles to consider, but that it is how the Group controls the individual.
Look up the BITE model at Freedomofmind.com if you have not yet. I would also explain that it is not only religion, but business, political etc groups who use these same methods to "corral" their members.
There is a thread somewhere, possibly here, where someone has taken the BITE model and compared it to the WTBTS methods so it makes it easy to see that the WTBTS is using the same methods used by other HCG.
Hope some of this helps.
NJY