A volunteer told news.com.au there were scenes of "absolute jubilation" among all those who walked into the bush to try and bring him home.
"We just saw Will," she said. "He looks in really good shape. He smiled at everyone as he was lifted out of the car."
He was located shortly before 1pm on Wednesday, just hours after his mother made a heartbreaking appearance on national TV.
"We are desperately hoping he will be found today," William's mum Penny told media this morning.
"He has an intellectual disability and, um, he's very smart in his own way, you know? I'm feeling positive because he's quite resilient. He is very skinny.
"He doesn't have much padding on him, but he eats all the time, he's always on the move. He's very active, so he's quite fit."
Despite two sleepless nights without her boy, Callaghan was optimistic that William would be found.
"As a mum, it's tough when I'm trying to sleep and he isn't," she said.
"He will be quite scared of loud noises ... I'm hoping he still has his clothes on. He may be barefoot, though. He doesn't like shoes. He will, you know, probably be covering his ears a fair bit if there is loud noise.
"Having said that, I think he's gonna be pretty hungry and scared, so, you know, food is gonna be a main driver for him at this point. And he will be seeking food. He loves his food, so he will be looking to — he's not gonna shy away from approaching someone for food.
"He won't ask them. He can't, he's non-verbal, so he won't... You know, he might just try
and grab it. Or he might grab them by the hand and try and lead them somewhere to say, 'Take me'."
On the third day of searching, police employed increasingly personal tactics to locate him.
He had spent two nights in near-freezing conditions described by police as "life-threatening".
Police changed their approach slightly, playing the theme music to Thomas the Tank Engine — William's favourite show — over loud speaker.
They also brought in one of William's former teachers to call his name in the hope he will recognise her voice.
The Victoria Police air wing shared infra-red images captured overnight showing dense bush surrounding Mt Disappointment — bush that is thicker than usual because of regrowth following bushfires.
"When you have someone out lost in the bush, we are fighting against time," Search and Rescue Squad Senior Sergeant Greg Paul told reporters yesterday.
