How does Thomas help Victor better understand his father as a person?
Victor wants to avoid Thomas because he tells the same stories over and over again. Thomas has a more spiritual Native American sensibility about him whereas Victor struggles between this heritage and a more modern cultural identity. But even though Victor is embarrassed by Thomas, he accepts Thomas' money with the caveat that he takes Thomas with him on his trip to Phoenix to go through his father's things. So, this is the first way that Thomas helps Victor reconnect with his father.
After they search Victor's father's trailer, Thomas tells him a story about his father. Thomas went to Spokane to have a vision. Victor's father found him and told him he would only get mugged waiting out there. Victor's father took Thomas to eat and then took him home. Thomas concluded that Victor's father was his vision. Victor's father took care of Thomas and this was the message he intuited from this: "Take care of each other." Thomas also tells Victor that his dad told him to look out for him (Thomas). This is part of the lesson of taking care of each other. It centers upon the notion of community. In this way, Thomas is a connection between Victor and his father.
Victor and Thomas are on the verge of adulthood. Victor struggles with his identity, but this trip helps him. Note the symbolism. He obtains his father's ashes with Thomas-Builds-the-Fire. They go to "Phoenix." The symbolism suggests that in reconnecting with his father, Victor rises from the ashes: like the mythological phoenix. In other words, he has a better understanding of his father and his community ("Take care of each other").