A 9-year-old Wellington boy who went missing overnight on Monday is covered in scratches from head to toe after spending hours walking through gorse, his mother says.

A man out walking his dog in the bush around Tawa found Xion Flaws yesterday morning after the boy had been missing for a gruelling 12 hours, wandering barefoot in just shorts and a T-shirt.

"Xion loves animals so he came up because of the dog," mother Beverly Flaws said.

"He wanted the dog and as soon as the guy saw him I heard he picked him up and carried him to the closest street."

Flaws does not even know the name of the man who found Xion in the bush, but cried as she expressed her gratitude to the unknown rescuer.

"Thank you for being so amazing and finding my son and picking him up and bringing him to the closest street instead of making him walk," she said.

Xion is autistic and has intellectual disabilities, and is also non verbal, but Flaws said it would have been obvious he could not walk very far himself.

Xion went missing on Sunday evening while Flaws and her family were having dinner at a nearby friend's house. Xion had been playing on the trampoline with siblings but was briefly left alone, in which time he wandered off.

He had gone missing before, but never for more than an hour. He has regular places he wanders to, including the local New World supermarket and a park, but has never gone into the bush before.

"It looks like he was just straight walking through gorse," Flaws said.

"His body is just scratched up from head to toe. It's all over his face, all over his ears, his legs. He's got little prick marks all over his entire body.

"I can't even imagine how scared he was being stuck in that and not being able to get out."

Flaws had started searching for Xion herself, but when the police became involved they told her to go home in case Xion went back there.

Flaws sat outside the house until 4 or 5am, both hoping for the best and fearing the worst.

"His body is just scratched up from head to toe. It's all over his face, all over his ears, his legs. He's got little prick marks all over his entire body.

"I can't even imagine how scared he was being stuck in that and not being able to get out."

Flaws had started searching for Xion herself, but when the police became involved they told her to go home in case Xion went back there.

Flaws sat outside the house until 4 or 5am, both hoping for the best and fearing the worst.

"[I had] the dreaded thought of wondering if he was still alive, if he had tripped on something and hurt himself and not been able to get back up.

"[When I] got the phone call that he had been found I instantly burst into tears. I'm tearing up even thinking about it. It was an emotional rollercoaster, really."

Flaws is still feeling overwhelmed from the ordeal, but said Xion is "back to his happy self".

Initially when he was found, Xion was "exhausted", and needed six blankets, a warm towel and plastic wrap to bring his body temperature up while he was in Wellington Hospital.

Flaws will be keeping him home from school this week to avoid his numerous cuts getting infected. He also has severe nappy rash.

She wanted to thank everyone who joined the search for Xion.

"Seeing everybody out there watching, the whole community and other communities come together to find him, it was overwhelming.

"I just want to thank everybody, everybody was so amazing. It was still early hours of the morning, I could still see people out looking for him."

Flaws had "so much appreciation to everybody for being so supportive, so amazing, and helping to find my son".