Being that Carl is Jody's father, the boy is bound to look up to him as his guiding and principal mentor. But (and that's a pretty big but), Carl Tiflin's not the most talkative guy in the world, so Jody has to look elsewhere for guidance on occasion.
Billy Buck, the ranch hand, is a much friendlier guy than Jody's father, and he steps in where Carl leaves off. He teaches Jody all about horses, and about loss, too. Plus, he's understanding when it comes to Jody's feelings, particularly when Gabilan dies, so it makes sense that Jody looks up to the guy.