Meanwhile his parents who live on a large and busy council estate in Blackpool, Lancs have been warned by a Crown Court judge they face jail.
The boy lived in the bunker for over a year. It contained just a mattress and a potty.
And to while away his hours alone he scratched pictures and words on the walls.
Det Constable Matt Normanton who investigated the cruelty case said, "I have never seen anything like it, the place was far worse than a police or prison cell."
"I hope I never come across anything like this again. It was inhumane, barbaric."
The boy would be locked away for longer periods as punishment when he was caught taking food from the fridge - usually raw meat and butter.
Meanwhile his mother, 32, and father, 41, would seat and watch TV as the boy was left padlocked.
His cell was a former coal bunker which had been bricked up so it became an extension to the semi detached property.
Outside it had been boarded up and the only exit was into the house through the door which was padlocked.
The alarm was eventually raised by the boy's school and when the youngster was questioned by police he told how he "lived in a box" and how it was "creepy"
He has boasted to social workers he has got his first toothbrush and can play in the park.
The bunker had one window which was sealed and when police went to the house the boy's potty was full.
The couple both pleaded guilty to cruelty charges at Preston Crown Court.
Judge Anthony Russell adjourned sentence for probation reports warning the accused they face jail terms.
They must not have any contact with the boy or any unsupervised contact with any child under sixteen.
One of the couple's neighbours on the estate said: "There is a high wall between us and we did not know what was going on.
"All the houses had the old brick built coal bunkers which became redundant when central heating was installed.
"Many were knocked down and others made into small extensions of the house."
"We cannot understand why it took so long for the authorities to realise something was going dreadfully wrong. It should be investigated."
Blackpool council has launched an inquiry into how the situation was missed.
