His eyelids were so swollen that the baby could barely open them.
Court documents stated that the attack happened at a house in the Waikato area on February 19 last year.
For legal reasons the man, his baby and partner cannot be named.
The man's then-partner was cooking breakfast and he took the baby into the bedroom after becoming frustrated with his continual grizzling - the result of teething.
Alone in the room, the man struck the baby repeatedly on his face and head and grabbed him by the chin and mouth to stifle his cries.
He then forced something solid in the infant's mouth causing a tear between his tongue and base of his mouth.
From the next room the baby's mother could hear her partner say: "f*** up, shut up you little c***".
She threatened to call the police as the attack on the baby continued.
Her partner then turned on her, threatening to burn her with a hot pan before pushing her towards
the ground and kicking her legs, hip and shoulder.
As she lay on the ground, he then fetched the baby and demanded she feed him.
In November last year the man pleaded guilty to a charge of wounding with reckless disregard for the attack on his baby son.
He also admitted a charge of assaulting the baby's mother with a weapon, and six further charges of male assaults female relating to her and another woman.
The sentence upset the baby's maternal family, with his grandfather saying it was "a huge joke".
It has also sparked outrage in the community.
Police have just confirmed they are looking into the sentence, handed down by Judge Denise Clark in the Hamilton District Court yesterday.
Detective Inspector Graham Pitkethley said police noted the decision of the court and were "now taking time to consider the outcome before taking any further steps".
"This process is ongoing and we're not in a position to comment further at this time," he said.
Earlier today Family First wrote to the Crown Solicitor and Justice Minister Andrew Little, asking for a review of the sentence and imploring them to appeal the outcome.
