Maybe an older version of the boy, like his future self. He comes back in the dream to show the boy what he can achieve in the future and gives him some advice on how to get there.
The obvious main character would be the delivery boy himself. He could be a young guy just trying to make some money to support his family or pay for his studies.
Well, since it's this boy's story, the main character is the boy himself, of course.
The giant boy himself could be the main character, of course. He's the one with the unique situation of 'voring' things.
Maybe the main character is a boy named Jack who is very interested in magic. He has been reading about magical transformations and one day, while trying to perform a simple magic trick related to flowers, something goes wrong and he turns into a flower girl. This forces him to face a series of challenges in his new form and learn more about himself.
It could be an ordinary boy who is tired of being weak or bullied. He decides to take matters into his own hands and become a muscle man.
A shy and nerdy boy could be the main character. He's always been into books and science. When he is turned into a girl, he has to use his intelligence to figure out what happened and how to get back to his original form, all while dealing with the social changes like fitting in with the girls at school or in his neighborhood.
It could be a regular boy named Tom. He's an ordinary kid with a normal life until one day he's struck by a strange ray that makes him giant. As he tries to deal with this new situation, he becomes the main character of this wild 'giant boy shrink story'.
Perhaps the main character is a boy named Sam. Sam is a bit of a troublemaker usually, but being home alone changes him. He realizes the importance of responsibility. He has to take care of his pet dog while his parents are away. And when some unexpected events happen, like a small fire in the kitchen because he tried to cook something fancy, he learns from his mistakes and becomes more mature in the process of dealing with these situations.
The main character, the boy, might be a bit of an outcast. He doesn't fit in well in his community, so he often retreats to the forest. There, he feels more at home among the trees and the animals. He could be a creative and imaginative boy, always dreaming up stories about the forest and its secrets.
The main character would obviously be the boy. He could be a curious and adventurous kid who is open to new experiences, which is why he might be able to handle being in a girls' camp.