Tim grows as a character from blind faith in his father and the Revolution into a teenager who is more in touch with his father's failings and the reality of the war.
Here is an excerpt from here at eNotes that should help you understand the progress of the character. You can learn more about Tim, Sam, and Life Meeker as well as other literary elements by visiting the link below.
"Fourteen-year-old Tim Meeker idolizes both his brother and his father. Tim narrates the story, and this perspective allows the reader to observe his initial innocence and his eventual perception of reality. Torn between his love for Sam and his loyalty to Life, Tim resists choosing sides. His desire to attend to the everyday concerns of his family, even during wartime, makes him highly believable. His character changes with time, especially as he recognizes that his youth does not make him inferior to his older brother. Tim quickly grows to maturity by taking charge of family responsibilities, unlike Sam, who abandons the family to fight for self-glorification."