Heidi Christensen says her son Daniel was staying with his father and stepmother in their home near Great Falls when he packed up two backpacks and left on foot.
Daniel's father Ken Newell and his stepmother Roz Newell called their local county sheriff, but there have been few leads.
Daniel Christensen weighs 100 pounds, is 5'7" tall and was wearing a blue hooded sweatshirt/jacket when he left his father's home, where he was spending the school year after arriving in April. He had spent more than a decade living with his mother in the Layton/Syracuse area before he was sent to live with his father.
Heidi told ABC 4 News that there had been two possible sightings of Daniel in Montana, where callers said a boy matching his description was seen in the company of a man with long grey hair and beard.
Daniel's stepmom said she received a call on Sunday from an unknown number with a bad connection in which a man's voice said, "Your son is dead."
Sheriff's investigators were trying to track down the source of the call.
Daniel's parents and stepmother have been placing fliers all over the Mountain West from Montana to Utah in the hope that someone has seen him.
Anyone with information that might help locate Daniel was asked to call Ken Newell at 806-395-0979 or 406-231-2766, or the Cascade County Sheriff's Office at 406-454-6846.