"I cannot ascertain that there was a crime on the part of the driver," prosecutor Daniel Brodén said in a statement.
The Akademiska sjukhuset hospital in Uppsala declared the boy dead in late February after he spent three days in hospital.
The child succumbed to extensive internal injuries after he had been found in the municipality of Kramfors, when a woman heard him screaming in a carpark.
Police initially classified the incident as suspected manslaughter, but changed their tune on Thursday after an investigation.
"The investigation suggests that this whole thing was just an extremely unusual and tragic accident," the Västernorrland wrote in a statement.
A truck driver working on the scene at the time of the incident was a suspect at first, but prosecutors were unable to tie any crime to the driver.