Make them show you proof for 1914, where they will show you calculation how they got to 1914.
This happened to me about 12 years ago when I was very active studying with a smart young college kid. After I had explained the JW view I felt like it may well be the dumbest load of crap that ever came out of my mouth. It was embarrassing because the kid wasn't buying it and JWs aren't prepared to deal with smart people who don't succumb to the programming. Another thing about it was that he asked about it fairly early on (like within the second month of the study, maybe), not late in the book where it is dropped on the victims after they're already mentally conditioned to accept everything. He didn't tell me anything, he asked and expected me to provide an answer.
Obviously this didn't immediately make me leave the organization (it was over a decade later, in fact) but it has to have been a significant early experience or I wouldn't remember it so vividly. I think the cognitive dissonance he caused by making me explain something in detail that doesn't stand to reason was one of the real successful blows to the blind faith in the WT organization that I was raised to have.
Also, in relation to 1914, once they explain the doctrine and how they get the number, assess how uncomfortable it made them and then a couple weeks later tell them you've been thinking about it and since you're interested in the historical accuracy of the Bible, you can push the point further by asking where in the Bible you can find the date 607 (which is their start-off point for counting down to 1914). When they explain that, and it sounds overly complex and unsupported, then you can hit them with the question of listing the kings of Babylon counting back from 539 to 607. All the information to do so is in the Watchtower Library CD-ROM, but the issue they'll run in to is that their kings' reigns only lead them back to 587, the secularly known date for the destruction of Jerusalem, not 607 the Watchtower manufactured date.
Anyway, ask questions but don't try to tell him anything. Black Sheep has excellent advice on this sort of stuff.