Larry thinks of Sonny as a prig partially due to his jealousy of Sonny. Part of the reason that Larry spends so much time acting out of line is because he perceives Sonny as his mother's favorite and thinks that he will always be second place. While Sonny is particularly fond of books and studying, Larry often cuts class, spends time with his rough friends, and dreams of being a soldier.
When Sonny begins to act on his best behavior during Christmastime, Larry continues to skip school. On Christmas Eve, Sonny reprimands Larry and tells him that there's no way Santa Claus would visit a boy who behaves as he has. Larry thinks that pointing this out is extraordinary priggish behavior, and it leads to Larry's plot to stay awake and swap the gifts.