There are two kinds of things that Hemingway reveals about the boy Schatz in "A Day's Wait." The first is his relationship with the people in his world. He is trusting because he never questions what he learned at school in France from his friends. He believes the trustworthiness and believability of the people he knows. He has an inexperienced and therefore narrow vision of the world (don't we all) based on his limited experiences.
The second is inner personal qualities and characteristics. Though Schatz has the experience and world-understanding of a boy, he has the courage and determination of a man: He courageously determines to meet his (supposed) fate without shrinking in fear. He has the personal integrity and strength of character of a man: Even in his (supposed) last hours, he thinks of his father's comfort and prefers to stay awake, presumably to meet death head on and not lose any experience of his short life.