The family of a four-year-old boy killed after a 4WD ploughed through a medical centre carpark in Melbourne's south-east have thanked the public for their support.

"Our darling Brax was tragically killed in a car accident yesterday. Our family thank the community, extended family and friends for respecting our privacy and for all your love and support," a statement from the Kyle family read.

The family said a private ceremony will be held with only intimate family in attendance.

Brax was holding his father’s hand as they left the Epworth Specialist Centre in Berwick yesterday, when the out-of-control Toyota LandCruiser ploughed into parked cars and pedestrians.

Investigators are probing how the 56-year-old Pakenham driver lost control, crossed the median strip and became airborne, taking out at least ten vehicles just before midday.

Witnesses said the man told them he had been on dialysis and had just left hospital, and police are investigating if he had become faint or dizzy behind the wheel.

The four wheel drive struck three people, including Brax. (9NEWS)

Emergency services at the scene in Berwick. (9NEWS)

Nurses from a clinic nearby desperately tried to save the young boy as his heartbroken father watched on.

The driver was taken to Dandenong Hospital in a stable condition.

Major Collisions Unit Detective Sergeant Mark Amos told reporters yesterday the father and son from Narre Warren South had just walked out of the clinic when the boy was hit.

"The father's gone out with his son, holding his hand, doing what good dads do," he said.

"There's just no way you could've predicted this could happen."

Cameron and Sarah Smith, who had been travelling behind the LandCruiser at the time, said they remembered the boy’s father saying “it’s my fault”.

“It’s not his fault – those poor parents,” Ms Smith said through tears.

The driver of the four wheel drive remains in hospital. (9NEWS)

Mr Smith ran up to the wreckage and tried to check on the driver, who was trapped inside the car.

"He was in front of us and he started to veer off into oncoming traffic and he's gone airborne," Mr Smith said.

"He stated that he had dialysis, he just had blood taken out and he just literally pulled (out) from the hospital."

A woman in her 60s was also struck by the vehicle and was last night recovering at Casey Hospital.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.

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