Well, if the boy is anonymous and is not known throughout the book than that allows the reader to experience a sense of making up the character in the readers image. Usually this kinda puts you as the boy if that makes sense. When the character is known than it gives the reader a different experience with imaging this character from a 3rd person perspective. The theme of it would basically just be that if or if you don't mention the main character determines if you imagine the characteristics of the character or not.
Hope this helps! XD :)