Bridge to Terabithia is written from the third-person point of view; however, the view is quite limited rather than being omniscient. Readers see and experience things as if they are looking over Jess's shoulder. The result is that they get to know Jess and his feelings quite well.
From early on in the story, readers get a strong sense that Jess believes he is the odd man out of his family. In chapter 6, readers will even get to see his thoughts about believing he isn't actually a blood relative of his family. Jess imagines that his father found him floating down the river in a basket and brought Jess home in order to have a son rather than only daughters. The image is reminiscent of the story of Moses from the bible.
By chapter 6, Jess has spent enough time with Leslie to know that other people like him exist. He has learned that there are people that like reading, art, and imaginative play. Many of Jess's sisters are cruel, self-centered, and materialistic. Jess struggles to relate to his father, who at first appears to have a narrow view of what boys should and should not be interested in. Jess realizes that he has far more in common with Leslie than he does with members of his own family, so it makes sense that he might think (or want to believe) that he must be from another family.