Flyboy of Underwhere by Bruce Hale

Sometimes even sidekicks have to step up.

Here's the thing: Zeke's busy being the so-called Prince of Underwhere. His prissy sister, Stephanie, is some sort of pirate queen. But Hector? Everyone treats him like a joker and a sidekick. Well, those days are over. Now only Hector can save the day—if he can survive the swarms of sharp-beaked midget flying dinos, smart-aleck flying horses, angry armies in their undies, a really, really bad hypnotist, and a duel with deadly toilet plungers. . . all the ordinary wedgie weirdness of the tighty-whitie world under our own.