Among other forms of abuse, the boy was subjected to beatings, electric shocks and being forced to lie naked in the snow.
The boy's mother, who was also released on parole this summer according to tabloid Aftonbladet, was convicted alongside her partner in 2006 of aggravated manslaughter (grovt vållande till annans död).
The pair eventually dumped their son Bobby's body in a lake after finding him dead in bed.
The disturbing details of the abuse shook Sweden in 2006.
Among other forensic evidence, Bobby's blood was found in the family's vacuum cleaner, which had allegedly been used on his private parts.
The stepfather had large amounts of pornography on his computer, many with elements of sadomasochism.
Much of the criminal case involved the parents blaming each other for the abuse.
Bobby's case also led to changes in Swedish law.
The Lex Bobby law, introduced two years on in 2008, outlines the responsibilities of people working with children to report suspicions of abuse.
After his death, Bobby's teachers reported that he spoke about sex more often than his class mates. He had also mentioned his mother's drinking.
No one followed up on the signs.
Despite the extreme abuse, Bobby's parents were not convicted of murder.
"It wasn't possible to prove that they wanted to kill the boy, only that they showed extreme carelessness," criminologist Jerzy Sarnecki told news agency 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."
It is customary in Sweden that convicted criminals serving long sentences are released two-thirds of the way through their sentence, he added.