Many whites lived as members of a tribe, was a common thing since whites first came here. So your Grandfather might have been a bona fide tribal member, considered an Indian, but still white.
Interesting story, When the the Lewis And Clark expedition first made contact with the Mandan tribe, they believed it was the first time white men had been in that area, the Mandan confirmed to them they were the first white men hey had met. Lewis and Clark were amazed that several members of the tribe looked white, they had red or blond hair, fair skin, blue or green eyes.
It caused quite a stir in the scientific community when the expedition was over and their find announced. Many believed the Mandan were the remnants of one of the "Lost Tribes of Israel", or perhaps the Roanoak colony. It turned out, French trappers had been living with the Mandan for at least 50 years before Lewis and Clark came along. That area was their best fur trapping ground and they had not told anyone back east about it. It was their children Lewis and Clark had seen.
Meh, I'm over half Choctaw and it never helped me see one.
Anyone born here in Native American, I prefer American Indian.