Update: The autistic teenage boy missing for almost three days on Victoria's Mt Disappointing has been found alive.

William Callaghan, 14, had been missing since walking away from family members on Monday afternoon.

He spent two freezing nights in temperatures that hovered just above 0C as more than 400 people combed the dense bush to find him.

He was located shortly before 1pm on Wednesday, just hours after his mother made a heartbreaking appearance on national TV.

Earlier: The distraught mother of an autistic boy missing at Mt Disappointment for two nights has described how her "resilient" and "very smart boy" would be managing in the bush.

William Callaghan hasn't been seen since wandering away from family during a hike at in the rugged bushland 60km north of Melbourne on Monday.

"We are desperately hoping he will be found today," William's mum 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."

She remains optimistic that William will 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.

On the third day of searching, police have employed increasingly personal tactics to locate him.

He has spent two nights in near-freezing conditions described by police as "life-threatening".

Police overnight 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.

William was last seen about 2.20pm on Monday. Family says he disappeared wearing only blue trackpants and a hoodie and that he was not dressed for the freezing conditions

"When you have someone out lost in the bush, we are fighting against time," Search and Rescue Squad Senior Sergeant Greg Paul told reporters on Tuesday.

William went missing without food and water and could have covered a lot of distance, police said.

It is believed he could have tried to walk into a house for food or put himself to bed, and police have asked neighbours to check their outhouses.

"William doesn't verbalise so if anyone finds him, the best way to communicate with him is to be patient and calm," Acting Inspector Christine Lalor said.

Family and friends have shared desperate messages asking for help as the search continued yesterday.

"Emergency request," Liam McIntire wrote on social media. "Our sister-in-law's cousin is missing. If anyone knows ANYTHING that could lead to his rescue, please contact the Victorian Police."

According to 3AW, Police were "a bit overwhelmed" by the number of volunteers who arrived at Mt Disappointment yesterday wanting to help.