His 11-month-old sister, who was also rescued from the burning dwelling, was transported to Maine Medical Center in Portland with injuries that authorities said were not life-threatening.
The parents of the children and an Oxford police officer who helped rescue them were treated for smoke inhalation.
The fire was discovered shortly before 2 a.m. at the Oxford Pines Trailer Park on Sheryl Lane when the parents, Kathy Bennett, 37, and her husband, Neil Gephart, 30, were awakened by a smoke detector.
Bennett and Gephart escaped through a bedroom window, then tried unsuccessfully to get back inside to reach the children, said Stephen McCausland, spokesman for the state Department of Public Safety.
A neighbor, Raymond Reese, was credited with reaching into the girl's room and pulling her crib close enough to a window to get her out, while Oxford police officer Alan Coffin entered the boy's room to get him out.
The boy, identified as Shawn Bennett, died at Stephens Memorial Hospital in Norway, McCausland said.
The infant, Andrea Bennett, was transferred to the Maine Medical Center, where she was in critical condition late in the morning. No other information was available, a spokeswoman said.
Crews from Norway, Mechanic Falls and Otisfield helped fight the fire.
Investigators from the state fire marshal's office determined that the fire started in the living room, but Sgt. Kenneth Grimes said investigators will need another day to try to pinpoint the cause of the fire.
