I remember reading a novel in middle school - I think the protagonist’s name was Jim. He was from a white family but he ran away to live with some American Indian tribe (this was set in the pioneer days - Little House on the Prairie sort of background). I enjoyed it immensely as a kid but I can’t remember the title at all. 
Are you thinking of The Sign Of The Beaver? That’s about a 13 year old named Matt who is left to guard the homestead while his father goes to meet his mother and sister. He befriends local Indians while he’s alone.
Actually I finally managed to find it - it was Moccasin Trail. 
Thanks for the suggestions though!