Police say Charles Edward Weimer, who's a prison guard from Lawton, was watching over the boy, his fiance's son.
Weimer and his fiance took the 2-year-old to Fort Sill's Reynolds Hospital last Tuesday.
Weimer claimed the boy fell down a flight of concrete stairs and was injured.
The boy died shortly after he arrived at the hospital.
Police say the only visible injury was a bump on the boy's head, but a medical examiner's report states the 2-year-old's injuries did not match up with Weimer's story.
Weimer is being held in the Comanche County Jail on a charge of second-degree murder.
He has pleaded not guilty and his preliminary hearing is set for March 12.
He could face 10 years to life in prison if found guilty of murder.
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