Facing mounting pressure in a battle with his estranged French wife over custody of their five-year-old son, a Belgian expatriate suffocated the boy by placing a cushion over his head and pressing it down, the High Court heard yesterday.

As banker Philippe Marcel Guy Graffart cradled Keryan Gabriel Cedric Graffart's motionless body in his arms, crying, he told him that he loved him and was going to join him.

The 42-year-old left his apartment in Leedon Heights and sped along the Marina Coastal Expressway (MCE) before deliberately crashing his car into the wall of the tunnel.

He removed his seat belt just before crashing but the car's airbags were deployed and, although he temporarily became unconscious, he survived.

Details surrounding the killing of five-year-old Keryan at the d'Leedon condominium on Oct 5 last year emerged when Graffart pleaded guilty to a charge of culpable homicide not amounting to murder, admitting that he had caused the boy's death at his 32nd-storey apartment between 9pm and 10.17pm.

At 10.41pm, he deliberately tried to kill himself, but failed. He was taken to Singapore General Hospital with minor injuries.

There, he called the police and said he had tried to kill himself. The officer told him to wait, but he left the hospital, took a taxi and returned to his apartment.

He contemplated - and decided against - other means of suicide, such as jumping off his balcony and stabbing himself, before going to the Bukit Timah Neighbourhood Police Centre at about 4.30am on Oct 6.

An officer on duty noticed Graffart looking disorientated, attempting to buy a drink from the vending machine just outside the police centre. The officer offered him a $2 note to purchase a can of Coca-Cola and saw Graffart walk away without collecting the change.

There were cuts and scrapes on Graffart's forearms, the needle of an intravenous drip attached to his right hand, and a hospital identification tag on his left wrist.

When questioned, he revealed that he had come from a hospital after trying to kill himself by crashing his car into a wall. He then told the officer: "I have done something really bad to my son."

When police officers went to his apartment, they found Keryan's body on the bed in the master bedroom. He was pronounced dead by paramedics at 6.17am.

An autopsy found bruises and abrasions on the boy's head but could not ascertain the cause of death. Zolpidem, a drug used to treat insomnia which is not recommended for use by children, was found in his blood.

Graffart said he had prepared two tablets of the drug for himself but allowed Keryan to take the medication after his son asked for one, thinking they were vitamins.

The Belgian, who was the head of fund distribution for the Asia-Pacific region with Nordea Bank, was initially charged with murder.

The charge was later downgraded to culpable homicide as he was assessed to be suffering from a major depressive disorder which substantially impaired his judgment.

The events leading to Keryan's death revealed a bitter custody fight between his parents, who are undergoing divorce proceedings.

The couple separated in 2014 and agreed to joint custody. But a tussle for Keryan arose after his mother was diagnosed with breast cancer in July last year.

She wanted to return to France with full custody of Keryan. Graffart suggested that she move to Luxembourg but she said no.

On the afternoon of Oct 5 - hours before the killing - Graffart received an affidavit pertaining to care and custody proceedings. It contained transcripts of conversations between him and his wife that she had recorded.

Distressed by the contents of the affidavit and pressed by his lawyer for a response, he told his lawyer that he was "so tired of all this" and wanted to stop the legal fight.

He did an Internet search for how long it would take to suffocate and choke, as well as whether one could survive a crash at 150kmh.

Before he smothered his son, Graffart told his domestic helper that he needed privacy, so she went to her room and remained oblivious to what was unfolding.

A Belgian Embassy representative was in court yesterday, as well as a friend of Mrs Graffart's, who would only say that she is in France.

Graffart will be sentenced on Aug 22. He faces a maximum of 10 years' jail, caning and a fine.