I don’t think being confrontational is really helpful at all. You’re just putting him on the spot to defend his preaching. I find I can really get people thinking by just posing ideas. One of my favorite things to say is, “I once heard that if you’re not preaching Christ, you’re preaching something someone from almost any religion could preach, which means you’re not really preaching a Christian sermon. What do you think?”
Ouch, LOL, wow, etc.
I do not recommend that, but that'd be something, wouldn't it? :-)