Police did not oppose the application by the man's lawyer for discharge without conviction.
Judge Patel said the man had thought another family member was taking care of his son, and in any case had driven slowly up his driveway, taking care to mitigate sunstrike, which had impaired his vision.
"Everybody at the house that day, including yourself, thought [the baby] was safe," he said.
A conviction would mean stigma not just for the man but for his family, Judge Patel told the court.
"A conviction would be a reminder that you caused the death of your son."
The man had no previous convictions and showed good character, Judge Patel said. He was supported by a loving family and had been seeking counselling to deal with the trauma caused by the accident.
"If you were to be convicted you would be a victim," he said.
"What happened that day was a tragic accident."
The 19-month-old was rushed to the East Tamaki Healthcare medical centre by family about 5pm on April 5 last year, but he died shortly after.
The boy was living with his paternal grandparents and parents.
One witness told the Herald at the time that he saw a woman wailing, surrounded by police, "on the cold concrete in Otara". Another saw the family outside the medical centre crying.
Some neighbours said they were so shocked by the child's death, they didn't know how to approach the family.
"We want to tell them how sorry we are, but it's not going to bring him back," one man said.
Witness Stefan Ruta also described the aftermath of the tragedy to the Herald.
He was leaving his own home when he heard a "big bang".
"That made me turn around. I saw the man running up the driveway. He was in really big shock. Another man jumped in the car and they went to the doctors.
"The mother, she was still in the house. Someone ran in to get her and she came out. She ran down the driveway and then up to the house. She was just in a total rush."
Another neighbour said the toddler's parents were devoted to him.
"Even though they are young, they are really good parents. You could see they loved their child."
Peter Uys, principal at Sir Edmund Hillary College, which the parents attended as students, told the Herald at the time he was shocked and saddened to hear what had happened.