During one night, Sinclair became "grumpy" and slept in the lounge with the 11-month-old.
"Something plainly went wrong," Judge Lynch said.
The next morning the youngster was heard crying and another member of the family "awoke the house screaming about [the child's] eye" which was swollen.
Sinclair said the child could have fallen off the coach or coffee table.
Later in the day, when bruises were noticed on the child's leg, Sinclair again explained it away by saying he fell.
Despite Sinclair's partner suggesting the baby be taken for medical treatment, Sinclair said he did not need it.
The boy was returned to his mother three days later having received no medical treatment, but Sinclair said the child had been seen by a family doctor.
It was not until the next day that the child was taken to the doctor, where he was then rushed to the local hospital then taken to Auckland's Starship Hospital.
During the trial, Sinclair blamed his son - the child's father - for the assault.
The doctor who treated the child at the time said the child's injuries were so severe that he could have died.
"Clearly he will never be the same," Judge Lynch said.
In a victim impact statement the child's mother said she had lost the first two years of her child's life.
"Because of you [my child] will never be the same."
A pre-sentence report said Sinclair still did not accept he assaulted the child, but he did accept he did not seek medical help soon enough.
In sentencing Sinclair, Judge Lynch said Sinclair had attacked the most vulnerable when he was alone in a room with him.
"Our humanity can be [judged] on how we treat our most vulnerable."
He also noted that Sinclair's family had placed great trust in him.
The suffering Sinclair made the boy endure was "nothing short of reprehensible" and Judge Lynch described Sinclair's actions as "cowardly".
The child's family did not wish to comment outside court.
Earlier this month Sinclair was convicted and discharged after he pleaded guilty to stealing credit card details from a woman he met online last year.
He then used the stolen money to pay for another female friend to visit from Auckland.