The boy was discovered at lunchtime on Friday in the municipality of Kramfors. He was found by a woman who heard him screaming and immediately rushed to hospital.
"He was not completely unconscious when he was found but was in a very bad way so it was difficult to get a clear picture of what had happened," Robert S Öhman of the Västernorrland police told the TT news agency.
After being transferred to a local hospital the boy was moved to Uppsala where his condition was described as "very serious."
"The injuries are substantial internal damage to the abdomen and pelvis which is consistent with the injury picture that he may have been run over by a vehicle," added Öhman to Aftonbladet.
The police officer went on to say that if the boy had been the victim of a hit and run, that the driver may not have been aware of the accident.
Police are particularly keen to get in contact with the woman who found the boy but left the scene before they arrived. They have also anyone who had parked in that area between 12:30 to 13:00 on Friday to alert them.
Despite a nationwide appeal for tips the police have few leads to work on with officer Öhman admitting the case is something of a mystery.
"It is very very unusual. We usually find out quickly what happened. It sometimes happens that people panic and disappear after an accident but they are always part of the latter," he said.
Colleague Mikael Bladh told Expressen; "We have fundamentally nothing to go on. We are fumbling in the dark."
The TT news agency reported that the boy's parents are currently at his bedside at the hospital in Uppsala.
Police have classified the incident as bodily harm.
Anybody with information is urged to contact the police in Västernorrland by calling 114 114 or via email on firstname.lastname@example.org.