Experienced local bushman Ben Gibbs spotted him and gave the non-verbal autistic 14-year-old a pair of socks, chocolate and a jacket before taking him to safety.

Despite his ordeal, William was seen smiling soon after his rescue, having survived almost 48 hours in dense bushland around Mt Disappointment, north of Melbourne.

He was taken to the Royal Children's Hospital, where doctors say he is in good shape and expect him to go home on Wednesday night.

"I am really overwhelmed here. He is as well as could be under the circumstances, he is quite calm considering," relieved mother Penny Callaghan told reporters soon after being reunited with her son.

"I can't imagine what he's been feeling and going through. I am just so relieved."

William's first request was for McDonald's, so it's on the menu.

"I think he wanted hot salty food," Ms Callaghan said.

She thanked all the searchers for their efforts.

"More than anything, thank you everyone. I'm so grateful, you're all amazing. What an amazing community," she said.

Mr Gibbs was one of hundreds of people who had searched for William after he disappeared on Monday afternoon.

Police drew on fresh tactics to draw him out, with his favourite tune, Thomas the Tank Engine, blasted over sound systems and food left out for him.

But Mr Gibbs, who grew up in the area and calls Mt Disappointment his "family mountain", dug deeper into the bush than others before him.

"He was just about 15 metres from me, just standing there, he was really angelic just standing," Mr Gibbs said.

"He looked in reasonable health, he wasn't shivering too bad, he didn't have socks on so I put some socks on him and a jacket. I gave him some chocolate. He ate half the chocolate bar."

The bushman let William eat and then carried him a bit before he walked out wearing Mr Gibbs' shoes.

"I am the one who stumbled across him, but everyone found him," he said, having felt like he was on the right track before spotting him.

"This is kind of our family mountain. I've been coming up here since I was a boy, so I know it really well," he said.

William was about 1.5km from the staging area - a 10 minute walk off the track in bushland, Acting Inspector Christine Lalor said.

"What an amazing result," she said.

"He is alert, warm, eating and drinking and he actually asked for McDonald's."

William's shoes were found about 100m away from the main track, fellow volunteer Carla Visona told media after he was found.

"I nearly started crying, it was crazy. Just the best feeling and the best outcome," she said, off to the pub to celebrate his return.

William was taken to the search staging area about 1pm and searchers were asked not to break out in the usual cheers so not to overwhelm him.

Within the hour, he left in an ambulance while eating Tic Tacs, joined by his mother and stepfather Nathan Ezard.

That's when a huge cheer erupted across the search camp.

Royal Children's Hospital registrar Dani Bersin later told reporters that the teenager is recovering well.

"He's got a couple of abrasions on his feet and one or two on his face but other than that he is looking pretty good," Dr Bersin said.

"We think he should be able to go home later this evening."

Deputy Emergency Medicine Director Joanne Grindlay said William did not get hypothermia, probably because he's an active youngster.

"Children are often a lot more resilient than us adults," she said.

Australian Associated Press