The 27-year-old father from the Adelaide suburb of Gilles Plains pleaded guilty in the Adelaide Magistrates Court to an aggravated charge of endangering life.
A separate charge of criminal neglect was dropped by the prosecution.
It has been alleged that the child had to be treated in hospital for malnourishment after being removed from squalid conditions at the man's home in October last year.
In a police statement released by the court, the man told officers he had been drinking vodka for three days and had slept for just three hours in that time.
He called himself a "stoner" and a "pisshead".
The man told officers he had a three-month-old child taken away from him by authorities in another state previously, and felt a repeat of that might have been avoided if he had family or friends to support him in South Australia.
"We seriously don't have anyone that cares for us, man. No-one cares for us. I think if we had one person that actually cared for us we probably wouldn't be in this position," he said.
The man described how he and his partner would lock the four-year-old in his bedroom each night because they feared he would flee.
But in October 2013, the boy was left in the room for 13 days, wearing the same nappy, and only given sachets of yoghurt and jelly on paper plates, slid under the door.
"We actually fed him pretty much every day, in that room, by sliding, sliding the plates under there, so as far as we could hear, he was happy," he said.
The court was told the boy weighed less than 10 kilograms when he was removed from the house.
The boy's 23-year-old mother will stand trial after pleading not guilty to her charges.
The child's father told police the boy's mother had used a CD as a mirror to look under the door and check the boy's welfare.
The man said the couple had planned on getting the child out of the room each morning after they had eaten breakfast, but they got "bloated and distracted" and never got around to it.
Statements from police have also been released.
They outline the appalling conditions in the home when they were called there to investigate reports the man had assaulted the woman.
One said the stench reminded her of the smell of a decomposing body and she "didn't know how people could live in these conditions".
The case has been sent to the District Court, where the man and woman will re-enter their pleas next month.
Read more: http://www.abc.net.au/news/2014-04-11/father-admits-failure-to-feed-4yo-son-for-13-days/5384422