Philippe Marcel Guy Graffart, who killed his son last October, was jailed for five years yesterday.
He had earlier pleaded guilty to culpable homicide not amounting to murder.
The Belgian, 42, was expressionless when Judicial Commissioner (JC) Hoo Sheau Ping read out the sentence in the High Court.
JC Hoo said that the penalty was consistent with similar cases and the accused's action could not be condoned.
"This is a tragic case in which the life of a five-year-old child has been ended by his father in the midst of a custody fight, causing immeasurable pain and suffering to all those left behind," she added
On Oct 6 last year, the police found the body of Graffart's son, Keryan Gabriel Cedric Graffart, in the master bedroom of Graffart's 32nd-storey apartment at D'Leedon Condominium in Leedon Heights.
Graffart had earlier told a police officer at the Bukit Timah Neighbourhood Police Centre that he had "done something bad" to his son.
The court previously heard that Graffart had smothered the sleeping Keryan with a cushion between 9pm and 10.17pm on Oct 5.
Graffart then tried to kill himself by ramming his car into a tunnel wall along Marina Coastal Expressway (MCE). He survived with minor injuries and was taken to Singapore General Hospital.
His son was pronounced dead by paramedics at 6.17am the following day.
A toxicology report found Zolpidem - a hypnotic medicine used to treat insomnia that is not recommended for children - in Keryan's blood.
Describing that night's events, Graffart wrote in a letter as part of his mitigation: "There (are) no words to say how sorry and broken I am by this tragedy. I lost my precious boy, Keryan, he was my life and my love, I was so proud of him. I never imagined this could happen."
Graffart was found to have been depressed over an ongoing custody battle with his French wife, Mrs Gwendoline Graffart, 40.
His lawyer, Mr Ramesh Tiwary, said the couple's relationship had deteriorated to such a level that Graffart began to feel he would not be able to provide Keryan with love and care.
His client had never touched or threatened Keryan before the night of the incident, he added.
"He had never warned or threatened that he would harm Keryan in any way," he said.
However, Mrs Graffart had applied for a personal protection order, and was granted an interim expedited order against Graffart on Aug 11 last year.
It restrained him from committing family violence against her and their son.
Mr Tiwary said Graffart had consulted four doctors but none of them had detected that he was emotionally and mentally unwell.
Mr Tiwary said: "The accused's (Graffart's) state of mind was such that he was clearly not thinking clearly and was very disturbed."
After the killing, an Institute of Mental Health doctor found Graffart to be suffering from a major depressive disorder.
Nevertheless, Deputy Public Prosecutor Sharmila Sripathy-Shanaz said Graffart's psychiatric condition "neither justifies nor absolves him" from committing the offence.
"No doubt, the accused's major depressive disorder contributed to his decision to smother Keryan... There is nothing to indicate that he lacked the capacity to comprehend his actions or to appreciate the wrongfulness of his conduct," she said.
A report dated Nov 23 last year by a doctor who had examined Graffart stated that he "was cognizant of the nature and quality of his actions".
Citing the jail terms of similar offenders in previous cases, the prosecution recommended a sentence of between five and six years' jail for Graffart, who had been assessed to pose a low risk of danger to society.
Read: Mum of boy killed by dad: My son 'was a tool for revenge'