The mother of Matthew Tsang, a 17-year-old schoolboy who was found alive after he went missing for a week, said her son had survived the ordeal thanks to his "strong will power".

The pupil at the Diocesan Boys' School was found at Ma On Shan Country Park on Wednesday afternoon.

On social media, his mother said the teenager managed to survive "using his strong will power and drinking water from the stream".

"After typhoon signal no. 9 and black rain, he was thoroughly soaked and realised that he might fall sick if he continued to wear his clothes, so he decided to take them off and hide in the bushes to shelter from the rain and wind," she wrote on early Thursday.

"Matthew's physical and mental conditions are stable, and is now staying in the hospital for treatment."

His schoolmates reacted to news of his rescue with joy.

"Our entire class jumped with excitement when he was found. This was really lucky," one told RTHK.

"He is in my grade. I am happy that he's found and healthy," another said.

The school's headmaster, Ronnie Cheng, appealed to parents in a letter on Wednesday to give time and space to Tsang's family and refrain from making speculative comments.

He also urged parents to pay special attention to their children's needs and emotional well-being, and not to hesitate to contact teachers or school social workers for support if necessary.

Cheung Tin-yu, a senior station officer at the Fire Services Department, said rescuers had deployed drones and artificial intelligence to narrow down the search area at the country park.

With technology, officers could spot features not seen on traditional maps, which only show basic information like the terrain, Cheung added.

"When we fly a drone and take images, we can immediately create a seasonal map and find spots with water that are not shown on the map. We could see from the images that there was water at those spots," she told an RTHK programme.

"We had targeted our search at places where we spotted seasonal streams, areas covered by foliage that could be used as shelter, or abandoned huts."