I told him I'd been doing some research about TTATT, and that ...
Resist the temptation to show your hand.It is better to appear naive than to try and impress him with your repository of anti-JW 'propaganda'
Write your questions down by all means, but don't ask them. The only questions worth asking a JW are the ones you already know their rote answers for, and how to expose those answers as fraudulent. If you don't know what his answer should be, research it in their literature, or here, whatever, just don't ask him.
If his answer isn't in keeping with WT doctrine, don't ask us if he was correct until you have sent him off to get the evidence to back up his assertion. It doesn't matter if he is correct. He needs a few 'wins' to keep him coming back, but if he is wrong, he needs to do the research to prove it to himself far more than he needs your reasoning for why you don't believe him.