The man had served six years of a 10-year prison sentence. It is common for Swedish convicts to serve only two-thirds of long prison terms, TT reported Monday.
The man and his wife, the mother of the dead 10-year-old boy, were convicted in 2006 of aggravated manslaughter. While evidence of abuse had been found, prosecutors were unable to prove the couple meant to kill the boy, identified only as Bobby.
The boy had been beaten, subjected to electric shocks and forced to lie naked in the snow. Investigators found his blood on the family's vacuum cleaner, which had allegedly been used on his genitals, TT reported.
"It wasn't possible to prove that they wanted to kill the boy, only that they showed extreme carelessness," criminologist Jerzy Sarnecki told TT.
"If the court had found that they meant to kill him, then they would have been convicted of murder, which entails life in prison."
The couple dumped Bobby's body in a lake after finding him dead in bed, TT reported.
Susan Sarandon 'very excited' about daughter's pregnancy
Interpol investigating stolen passports on missing flight