He beat the boy to death and then lied about it to the child's mother.
On Tuesday, July 25, Iheanacho was jailed for life and was told he will have to serve a minimum of 18 years behind bars.
Sentencing him, Judge Mark Dennis QC told Iheanacho he had a "deeply entrenched character flaw" that leads him to overreact.
Iheanacho was in a relationship with Lilya Breha, and on November 20 last year, he took Ms. Breha's son, Alex Malcolm, to a park in Catford, south-east London.
A witness heard Iheanacho "raging" at Alex after the boy lost his shoe.
She could not see what happened next, but she heard booming noises as Alex was being punched.
The witness then saw him bending over Alex and whispering to him. Another witness said he heard a "child's fearful voice saying sorry," Woolwich Crown Court was told.
Iheanacho then rang Ms. Breha, lying about what happened.
"He's fallen asleep and he's heavy," he told her, before bringing the boy home in a minicab.
When she tried to call an ambulance, Iheanacho kneed her in the chest, jumped on her back and tried to choke her.
Eventually an ambulance was called and Alex was taken to Lewisham Hospital, where he died two days later.
"Iheanacho subjected that poor little boy to a brutal assault after flying into an uncontrollable rage just because Alex lost one of his shoes," said Detective Chief Inspector Tony Lynes.
"Afterwards Iheanacho came up with various stories to try to cover his tracks, insisted his girlfriend lie for him and attacked her when she tried to get medical help for her unconscious son," he said.
"It is no surprise the jury easily saw through his stories and while nothing can bring Alex back, I hope Iheanacho's conviction today provides his mother and father, and their families with some comfort," added Det Chief Insp Lynes.
"Alex was so small but he was my strength and my purpose for living," said Ms. Breha.
"The hardest thing I have ever had to hear, was that my child died. I remember it like it was yesterday. Lying next to him in a hospital and praying that everything would be fine, that he will open his eyes. I didn't even get to tell him I love him. All I got was to put my hand on his chest and feel every single one of his final heartbeats," she added.