Constructive criticism time: At the least it's not how I would have done it, instead of citing wiki, which may make eyes glaze over, I would have detailed harmful acts committed by "real cults" by which I mean those cited in watchgroups, like scientologists, JW's, and branches of mormonism like with the child molesting prophet that's in jail now. I would've pointed to jonestown types, the real nutballs. Instead of merely mentioning Scientology, mention the specific kinds of acts they do that make them cult-like, the absolute control in decision making, the gradual doling out of church doctrine instead of giving it all at once, the telling them to isolate themselves from people that don't agree, divorcing family members and friends from your life because they're labeled "suppressive" (ironically), the harassment they get when they themselves try to leave...
And with this I'd push the theme that he refrain from such passive use of that word as it makes it lose it's power and it's real meaning. If everything is a cult than who cares? You're a cult, I'm a cult... It's like: if everyone is a "Nazi" than who cares about "nazis." It does the word a disservice, and does those that suffered under them a disservice. Size is moot in my opinion, entire nations have become cults under their dictators, Hitchens would have referred him to North Korea (and I've heard it said by others the largest cult in the world simply put, is China).
For specific examples of cult action and harm, I would recommend checking out Steven Hassan, an ex-cult member (moonies) who now tries to work getting people out of them his webpage is
http://freedomofmind.com/index.php They offer some detailed definition of a cult. There's also lots of online stories of people victimized by cults like scientology, xenu.net is the first place I can think of to start... so yeah, research them for instance of real harm and what really makes a cult before you re-engage, and tell those stories. Stories are the interesting stuff and what he'll listen to...
Also, I'd try to shift the conversation away from Calvin, as that is indeed seeming to be a dead end. Maybe just ask him if he really thinks it's matters that much, whether he really is invested in it being true or not. The most recent turn around I got from someone over discussing a crackpot theory, who was really starting to seem hostile and emotional about it, I simply said "In all, I'm getting an impression of hostility from you, which I'm not sure of the reason for...did you have a lot invested in this guy's theory?"
(Oh btw, I must add, I didn't know it was you at first, I actually subscribe to your blog...it feels weird giving you advice...
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