T.J. Heller, 33, had wept Thursday as he read a statement apologizing to the child's mother, her family and his own family for the pain he has caused.
His trial was told Heller and the little boy's mother had moved to Medicine Hat, Alta., in June 2011.
Court was told injuries began to show up on the boy when the mother got a job and Heller, who was unemployed, became the child's primary daytime caregiver.
When he died three weeks later, an autopsy found the child died of multiple blunt-force trauma injuries to his head.
Heller was originally charged with first-degree murder, sexual assault and interference with a dead body, but those charges were dropped.
On Friday, the little boy's mother was tearful as the judge listed off the many injuries the child suffered at Heller's hands, but Heller maintained a stoic demeanour.
After court was adjourned, the mother cried out: "He didn't even get 18 years to live!"
Justice Rodney Jerke said there were many factors that weighed against Heller but he also felt the accused was genuinely remorseful for his actions.
The boy's father said after the sentencing that he blames himself for what happened because he wasn't there for his son.
He said he hopes to now become a grief counsellor so he can help other people deal with their own losses.