We don't live in Chik-fil-a territory, but yeah, I'd say "trying" all those is a must. Much like giving a curious kid a cigarette, maybe you should let him order as much as he wants.
McDonald's - Fries, yeah. I'd argue Big Mac also, though more for the sauce and total package. It's of course a pretty pitiful burger. Nuggets with sweet and sour and BBQ sauce are also a best in class.
Burger King - Whopper. Everything else is garbage (especially the sides), but a hot, fresh, well prepared Whopper is delicious. Personally, I prefer it without cheese as it's a mayo-dominant sandwich.
Wendy's - Single with cheese: example of a
good corporate fast food burger. Maybe consider the bacon variant instead, but I think it's overkill.
Taco Bell - Personal opinion, but I live for the beef burrito supreme. It's got the lettuce and tomato, guac, sour cream, refried beans, and red sauce. Most of the rest of the menu is fodder for poor decision-making while inebriated.
KFC - I could see passing on this one unless your local franchise is really, really good. I crave some simple original recipe every now and again, but I haven't had truly great KFC in ages. Oh! The macaroni salad though. The macaroni salad is worth it, if it's the same as the Canadian version. Otherwise, take him to a Popeye's.