Volunteer bushwalker Ben Gibbs found the 14-year-old, who is non-verbal, and offered him some chocolate, before alerting authorities.
A police helicopter then spotted the teenager from above, where he was in bushland among the mountainous terrain.
His parents, including mother Penny Callaghan and step-father Nathan Ezard, then quickly moved in to rescue him.
Ms Callaghan told reporters in a heartfelt interview that she could not believe her son had been found.
“I am really overwhelmed,” she said.
“He is as well as can be expected.
“He’s quite calm considering.”
However, she said William had used hand signals to communicate that he was “confused” and “scared”.
“I can’t imagine what he has been feeling and going through,” Ms Callaghan said.
Police said William was found by Mr Gibbs at 11:55 am around 1.5 kilometres from the Mount Disappointment base camp and 10 minutes from the main bush track.
The teen was then taken into a tent to be treated by paramedics after being driven back to the base camp with his parents.
He has now been taken to the Royal Children’s Hospital for observation.
