In Holes, why is X-Ray considered the boss?

The book does not explicitly explain why X-Ray is the leader of the boys; however, readers are given enough evidence and characterization to make some decent guesses as to why the boys follow X-Ray's lead. In general, X-Ray is good at reading people and situations. This is especially true when it comes to his ability to handle adults. Consequently, X-Ray has a wonderful ability to defuse potentially heated situations, and that often protects the other boys from the Warden and the counselors. For example, when a fight breaks out between Zigzag and Zero, X-Ray is quick to try to defuse the situation by running interference.

"'They just got a little hot, that's all,' said X-Ray. 'You know how it is. In the sun all day. People get hot, right? But everything's cool now.'
'I see,' the Warden said. She turned to Zigzag. 'What's the matter? Didn't you get a puppy for your birthday?'
'Zig's just a little hot,' said X-Ray. 'Out in the sun all day. You know how it is. The blood starts to boil.'"

X-Ray is trying to protect the other boys, and the boys recognize this and appreciate the fact that he's putting his neck on the line for them.
I don't want to make it seem that X-Ray retains his power through complete altruism. He does maintain his leadership position through a rewards system. Be nice to him, and he's got your back. If you are not nice to him, then he turns hostile. This creates a situation in which it benefits the boys to do things for X-Ray in order to not only avoid hostility but to also earn rewards.

All of the boys at Camp Green Lake in Texas have nicknames. Stanley is Caveman, Ricky is Zigzag, Lewis is Barfbag, Theodore is Armpit (the largest boy in size), Alan is Squid, Jose is Magnet, Rex is X-Ray (the second smallest in size), and Zero is just Zero (the smallest in size). The book states that Zero does not have a real name. Mr. Pendanski says it is "because there was nothing inside his head." In chapter 11, X-Ray claims that his name is pig latin and stands for the name Rex. X-Ray claims that he can see inside people. For example, he tells Mr. Pendanski that he has a "big fat heart." His nickname is also ironic; X-Ray confides in Stanley that his eyesight is not very good. He claims that he is "too blind to find anything" (p. 53). Although X-Ray is the second to smallest, he is the leader of the group.