You have to lower yourself and operate at his level.
Take him a recipe of a pale beer, just some base malt and maybe one specialty grain and one type of hops.
When he fills the grain bill ask for a yeast recommendation and stare him in the eye without blinking.
When he ask what type of yeast then say "the kind for this recipe, you're the expert and I trust your judgment". <don't blink>
If he recommends an ale yeast, say "but this is an lager recipe" <don't blink>.
If he recommends a lager yeast...well you know what to do by now.
Then say "Man, I'm disappointed, I thought you knew all about the proper styles but now I've lost confidence. I'll make my own decisions from now own".
It's called coming off the top rope. Slapping someone with out actually touching them.
Tubercleize the man.