What lesson(s) is the main character learning throughout the text?

This is a thematic question. There are many things Arnold learns. I think the biggest thing is that people are people regardless of race. Junior finds sympathetic and unsympathetic people both in Rearden and on his reserve. Check out some of the themes below. They are also lessons that Junior learns.

https://www.gradesaver.com/the-absolutely-true-diary-of-a-part-time-indian/study-guide/themes