When the police found him in a house in Adelaide, Australia, the four-year-old boy weighed just 8.3kg.
A more typical average weight for a four-year-old boy would be around 15 or 16 kg.
The police were lucky to find him, because his mother had called them to say that her partner was assaulting her.
The boy was close to death when he was found in a small room full of soiled nappies, Mail Online reported. He cannot be named for legal reasons.
The 24-year-old mother allegedly starved him, while she posted pictures online of her attempt at cooking a roast lamb and a melted chocolate dessert, Australian TV channel Channel 9 reported.
The District Court was told that the parents had been on a binge of alcohol and cannabis, and had fed the boy yogurt, custard or jelly on plates they slid under the door.
Officers had to walk through garbage bags, rotting food, dirty nappies and sanitary pads to get to the boy, who was surrounded by used nappies, plastic plates and his own faecal matter.
The mother told the police that she was "too busy having sex" with her partner to care for her son.
In an earlier police interview, the father, 28, said the boy was kept in his bedroom for 12 days.
"We actually fed him pretty much every day, in that room, by sliding the plates under the door. So far as we could hear, he was happy," the boy's father said.
Doctors said the boy's dehydration level was at 10 per cent and he was just days away from reaching a critical level of 15 per cent.
The mother was quoted as saying: "I've messed up, I've done the wrong thing. I know I shouldn't have thought about my man before my son.
"I didn't associate with my child like I should have, instead I was too busy having sex with my partner."
The couple pleaded guilty to an aggravated charge of endangering their son's life and prosecutors called for a lengthy jail sentence. The couple will be sentenced in December.
The District Court in Adelaide heard on Tuesday that the boy, who is in foster care, still hides food in his pockets, hates doors being closed and is not toilet-trained.
Since being removed from his parents a year ago, the boy has had a significant number of decayed teeth removed and has limited language skills.
A victim impact statement, compiled by authorities and the boy's foster parents, was read in court on Tuesday.
The boy's mother cried uncontrollably as it was read.
The report said that normal family outings were obviously new to the boy, who would not come out of his room for the first six months after his removal.
When he woke up, he would lie in his bed waiting for someone to come to his room.
This article was first published on October 30, 2014.
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