IOW, he can eat a dish with citrus juice in it without the slightest ill effect (as I can eat things with garlic powder in them), but he's in whatever degree of trouble you want to give him if lemon ZEST is in the dish.

Hmm, that would mean he can drink Mountain Dew (lemon juice) but not Earl Grey (orange peel, I think?). Well, that's okay, Mountain Dew has more caffeine anyway.