"He had a craniectomy, which is where they remove part of the skull to relieve pressure [on the brain]."
Mr Booth said he didn't know exactly what had happened in the accident, but that Tyler may have chased a ball out on to the road.
The driver of the car was believed to be travelling slowly.
"We don't think there was speed involved or he wouldn't be here," Mr Booth said.
He said he was concerned for the car driver, who was probably deeply distressed by the incident, and wanted to find a way to get in touch to assure the person it was simply an accident, and the family did not hold animosity towards them.
Mr Booth said Tyler's family needed to stay in Auckland to care for him, but had some financial concerns as Mr Booth is recovering from an injury himself, and is on crutches and unable to work due to a dislocated knee.
A family friend has set up a Givealittle page to assist the family during the difficult and traumatic time.
Mr Booth said he was extremely grateful for the support, and that any donations would be used to help with any medical costs and to enable Tyler's parents to be with him.
"Anything left over we will donate straight back to Starship or Ronald McDonald House," he said.
He said he couldn't emphasise enough how amazing those organisations had been in supporting the family.
If you would like to help support the family please visit www.givealittle.co.nz/cause/please-help-our-little-buddy-tyler-and-his-family