A short time later, he was carried into a waiting ambulance wrapped in a white blanket by his stepfather Nathan, with his mother following Penny close behind.

Volunteer Ben Gibbs, from Research, found William standing in the bush not far from the summit, with his hands over his ears, blocking the noise of a helicopter above.

"I came up from the bottom of the mountain, there is a single track there I know quite well. I just followed that up and went off track once I got near the top and saw where the guys had tagged where they searched previously and I went a bit deeper than that," Mr Gibbs said.

He said it was 20 minutes from the track, relatively near the summit.

"He was really angelic, just standing there."

Mr Gibbs said he tried to relax Will and speak calmly to him, before giving him some chocolate, socks and a jacket.

"I heard he liked Thomas the Tank Engine so I talked to him about [the character] Diesel.

"After he ate half the chocolate bar, I carried him out."

Acting Inspector Christine Lalor said it was "great news" and they would be providing more information later on Wednesday.

Almost 500 people in teams from the police force, SES and CFA searched for Will around the clock since Monday.

Many concerned members of the public donated their time to join the effort and police were forced to turn some volunteers away.

The mood changed at the search site about 12.30pm as word filtered through that there had been a potential sighting of Will in the bush.

At 12.46pm, acting Inspector Lalor confirmed the boy was Will.

Huge grins crossed the faces of volunteers and searchers when she announced the news, as friends of the family began crying tears of relief.

Police had asked the hundreds of people waiting at the search site to be quiet and not to cheer or clap in case it spooked Will.

The teenager, who is non-verbal and taps his chest to communicate, went missing on Monday afternoon when his family lost sight of him.

On Monday night, temperatures dipped toward zero degrees. On Tuesday night, the mercury dropped below 5 degrees.

On Wednesday morning, his mother, Penny Callaghan had described her son as a beautiful, gentle and resilient boy, and spoke of her desperate hope that he would be found safe.

"We are desperately hoping he will be found today," she had told reporters.

"Sometimes being the mother of an autistic child is really tough. I have two boys with autism, Will is my oldest son and he would be considered very low functioning. He has an intellectual disability," she said.

"He is very smart in his own way, I’m feeling positive as he is quite resilient. He is very skinny ... but he eats all the time, he’s always on the move so he’s very fit.

"He’s such a beautiful person, he wouldn’t harm a fly he is very gentle."

It was a unique search for the teenager, who had never spent a night outside by himself or been camping.

Will is a fan of Thomas the Tank Engine, so three speakers moved around the area playing the Thomas theme tune, in an effort to draw him out.

Police encouraged people in the area to cook a barbecue, if possible, as Will loves the smell of onions and bacon. He's interested in water bottles, so police also asked people to put water on their verandah or porch.

And they urged locals to open any windows and doors if they were cooking, in the hope Will would smell the food.

Authorities knocked on the doors of residents from towns and suburbs near the summit of Mount Disappointment to see if Will sought refuge in a bed or outhouse.

Searchers on foot, motorbikes and on horseback called out "Will" as they made their way through vegetation so thick that made it difficult for air units to search from above.

More to come.