The boy was declared dead on arrival at the University Hospital in Linköping, 200km south of Stockholm, on Monday. An official autopsy found that the boy had died a violent death.
The man, described as a close friend or relative of the boy, has admitted to police that he used violence against the boy, according to a press release from the Östergötland police force.
The boy was badly injured when he was found on Monday.
“These were not injuries he could have inflicted on himself,” said police spokesman Tony Georgson to TT.
Police say they will await a full autopsy report before proceeding further with the case. The 24-year-old suspect was arrested on Wednesday afternoon. He is suspected of murder, or serious assault and manslaughter.