A massive police search was under way last night for a teenage boy who disappeared in rugged bush with no water as the temperature soared to 42C.

Police said Dennis Dear, 14, who was last seen on a quad bike along a bush trail near Flint about 140km south-east of Perth, had not had a drink of water since 4am.

But last night, his bike was found in a big ditch about 8km south-east of the search command post.

The bike's chain was broken and the keys and the boy's helmet and gloves were missing.

The discovery allowed more than 80 State Emergency Service and local police to refine their search to a smaller area of bushland.

The police helicopter was also called in to search, using its night vision.

Search co-ordinator Ray Anderson said searchers would continue patrolling bush tracks in their vehicles throughout the night, with visibility too low for officers to search on foot.

The teenager's mother Kaye and father Peter remained at the command post last night.

Family members rushed to the area yesterday, with Ms Dear revealing her son was autistic and has cerebral palsy.

"He has a little bit of autism and a little bit of cerebral palsy," she said.

Dennis, Mr Dear and a group of friends arrived at the popular camping and biking site off Metro Road about 6am yesterday.

It was one of the first trips Dennis and his father had taken together after a seven-year separation.

His father realised Dennis had disappeared about 6.30am and began searching.

He was quickly joined by other bikers but they failed to find him in an hour-long search. The official search started after authorities were notified about 7.20am.

Police Acting Insp. Tony Colfer said grave concerns were held for the teenager, who was at severe risk of dehydration.

"We're throwing everything we've got at this," he said.

"You've got to look at the weather. The young fellow last had some water at 4am. It's a huge concern for us."

Ms Dear, of Kewdale, feared the seriousness of the situation would not register with her son.

"It wouldn't faze him at all as such," she said. "Even when he sees the search, he wouldn't realise it's about him."

She was hopeful her son, who is solidly built and about 180cm tall, would be able to survive the night.

"He's done a lot of Aboriginal learning about native plants, animals," she said. "He wouldn't have a problem, I'm sure of it."

Ms Dear said it was out of character for her son to leave his friends and family and go off on his own. Dennis did not have quad bike experience.

The West Australian

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