A nomad is a person who wanders from place to place. Sometimes nomads move according to the seasons.
In this case, Junior fits the definition. Rowdy called him a nomad with the intention of being derrogatory. He wanted Junior to feel bad for leaving the Rez behind and moving into white culture. What's interesting is that Junior moved to Reardan High according to a season of his life. Attending a public school off the reservation would compliment his ability to be educated.
Maybe Rowdy was jealous of Junior's escape, but the put-down certainly fits their relationship. Rowdy wants to see Junior maintain the culture of Native Americans, even though the picture of Rowdy's life given to the audience by the author is not particularly positive.