Police were so concerned about the welfare of the child, who they found staggering in the street in Hull, that they called an ambulance.
Tests showed he had a blood alcohol level of 187mg - more than twice the legal limit for driving.
On Friday Hull Crown Court heard how the mother, who was also heavily drunk, was seen arguing and swapping drinks with her son on May 7 2011.
The mother, who cannot be named for legal reasons, pleaded guilty to child cruelty.
Prosecutor Stephen Robinson read to the court a statement from witness Rachel Bibby, who heard the commotion.
Mrs Bibby, a clerical officer, said: "The boy was staggering around all over the place and was almost unable to stand up of his own accord.
"He was waving his arms in the air and fell onto the floor at one stage.
"I saw the lad with a large, clear bottle of what appeared to be a bright green alcoholic substance, possibly a flavoured vodka shot.
"I saw him having an argument with his mother as she wanted the bottle of alcohol, but he refused to give it to her and continued drinking from it."
Mrs Bibby said the mother then went into a shop and came out with a can of lager and proceeded to persuade her son to swap the bottle for the can.
Mrs Bibby said: "She took the bottle off the lad, opened the can of lager and gave it to her son. He had a mouthful, decided he didn't want it and began to shake the liquid in his mum's face.
"She shouted at him, grabbed him by the shoulder and poured the remaining lager over his head."
Mrs Bibby said she feared for the boy's safety and called the police.
She said: "Unfortunately, I made eye contact with her whilst on the telephone and I heard the female clearly state in my direction 'we always get drunk together and he always acts like this'.
"My main concern was for the young lad due to his age and the fact he was drunk, staggering around in the street, wandering into the road when he could quite easily have been run over and killed."
Prosecutor Mr Robinson said the boy had been pulling a red toy fire engine and had "tried to sit down on the toy, but fell off it".
"The police attended and PC Walker found the boy so intoxicated he could barely stand. The boy swore at his mother. He then slumped to the ground. He was crying and began to vomit."
Mr Robinson added the woman said she had not done anything wrong, and accepted her son had been with her since 9am that day.
Defending, Paul Genney, said the mother had "her fair share of problems".
He said: "She has had a wretched life."
Sentencing, Recorder Neil Clark told the woman: "This is a very serious matter.
"You were drunk and your son was exceptionally drunk, so drunk that he was detained in hospital. Many people would say you should go to prison.
"But the reality is that you have had a wretched life. It seems to me that examining what has gone wrong and addressing it is the better option in this case."
The child has been taken away from his mother and the pair are no longer in contact. The woman received a 12-month community and supervision order, designed to tackle her problems.
What a shocking and sad tale. Let's hope this child and the mother get some help.