Gary Davis, 33, from Leyton, east London, was also found guilty of cruelty to the boy and to his eight-year-old brother.
The jury at Nottingham Crown Court cleared the boys' mother, Samantha Jane Tuff, 30, from Leicester, of manslaughter. She had earlier admitted two charges of cruelty to her two sons.
The court had earlier heard how the brothers - who cannot be named for legal reasons - lived with Davis and Tuff in the St Matthew's area of Leicester.
They were subjected to violent beatings and extreme punishment in the name of being taught "discipline", the juryheard.
Davis was described by Tuff in a statement as an "uncontrollable monster".
Joan Butler QC, for the prosecution, said that on 17 February this year the four-year-old boy spent the night in a cupboard as punishment for not being able to sleep. After Tuff went to work, Davis made the boy wear several layers of clothing and run about "till the sweat was dripping off him".
Davis hit the boy with a cricket bat if he did not lift himself high enough while jogging, she said. He then forced the child to get into a cold bath.
Paramedics were called after the boy lost consciousness and the child was initially taken to Leicester Royal Infirmary suffering from hypothermia.
He was later transferred to the city's Glenfield Hospital for specialist care, but died.
A post mortem examination revealed that the child had suffered weight loss and extensive scarring and bruising.
"The liver had an internal tear and there were bruises to the small and large bowel due to a punch, kick or stab in that area," Ms Butler said.
The court heard that Tuff was aware of - and sometimes took part in - the abuse, and failed to do anything to stop it.
Violent scenes, with members of Davis' family hurling abuse at Tuff, erupted in the public gallery as the jury of six men and six women delivered the verdicts, reached after seven hours of consideration at the end of the three-week trial.
Jim Harding, the chief executive of the National Society for the Prevention of Cruelty to Children, said: "The horrific level of sustained torture these children suffered was on a scale rarely seen even by experienced staff at the NSPCC."
He added: "Our 150 nationwide protection teams help around 8,000 children a year but, thankfully, very few of their cases ever fall into this category of extreme brutality."Reuse content