After almost five years and three Supreme Court trials, a Perth man has been found guilty of killing his three-month-old son while he was hooning and speeding on the Kwinana Freeway in Perth's southern suburbs.

James Farmer, 25, was charged with manslaughter not long after the crash in April 2018, which also seriously injured his partner.

The baby was unrestrained in the car, and he suffered fatal injuries when he was thrown out of the vehicle when it slammed into a power pole.

The Supreme Court was told that before the crash, the car was seen by witnesses doing burnouts and travelling at excessive speed.

Three times legal limit

A sample of Mr Farmer's blood taken about three hours later recorded a blood alcohol content of .142 per cent, which is almost three times the legal limit.

The jury deliberated for several hours before finding Farmer guilty of manslaughter and dangerous driving causing grievous bodily harm to his partner, who was the mother of the child.

It was the third time Farmer faced trial.

The first hearing was in May 2021, but it was aborted after a legal issue arose.

He faced a second trial last year, but it ended with the jury being unable to reach a unanimous verdict.

The third trial was conducted over the past two weeks.

Farmer denied he was behind wheel

Farmer denied the charges, with his lawyer suggesting it could not be ruled out that the person who was actually driving was his partner.

She did give evidence at the trial, but maintained because of the injuries she suffered, she had no memory of what happened.

Other witnesses testified that when they arrived at the scene, they heard Farmer saying: "I f....d up, I f....d up."

Farmer was remanded in custody, and he will face a sentencing hearing in late March.