Answer :

"Rex Alvin Washburn (nicknamed X-Ray) is the understood leader out of the boys in Camp D in Camp Green Lake. He is apparently the leader, as he always gets front in line when the boys line up for Mr. Sir or Mr. Pendanski (nicknamed Mom) to fill their canteens. He also has what he says is his own special shovel, which he says is very slightly shorter than the others."  I got that from one of my friends that has read the book before. 
X-Ray is the obvious leader because he gets all the boys to follow his directives. He assigns names to the boys, decides what jobs they get and who works for him. X-Ray keeps on being the leader although he is smaller than many of the boys because he uses a system of allies and rewards.