Cho-sing admits that he was obese at the time - weighing 102kg - and was struggling to make new friends at Bethel High School, in Yuen Long, in the New Territories, because he had low self-esteem.
His mother encouraged him to try to get fit by trying his hand - and feet - at Muay Thai kickboxing and Sanda, or Sanshou, a Chinese hand-to-hand martial art. All the regular exercise helped him to lose weight - he now tips the scales at 72kg - and leave him with a highly toned, muscular physique.
Yet the transformation also uncovered Cho-sing's natural talent for both Muay Thai kickboxing and Sanda; within a year he was winning local Muay Thai tournaments.
Those successes led him to being selected to compete for Hong Kong in Sanda, at last month's Asian Junior Wushu Championships, in Shanghai. Although he lost in the semi-final to a rival from Afghanistan, he still came home with his first bronze medal from an international event. "My semi-final opponent and I were evenly matched - almost 50:50. It was just a pity that I couldn't make the final," says Cho-sing. "Maybe one of the reasons I lost was because I lack experience at such big events."
His next goal is to qualify for next year's World Junior Wushu Championships and the 2014 Asian Games. "I want to get stronger and improve so people around the world get to know my name," he says.
Cho-sing, who was born in Zhuhai, Guangdong, first came to Hong Kong when he was three. He spent most of his childhood on the mainland, but gained his Hong Kong ID card when he was 11. He is not a permanent Hong Kong resident, but moved here with his family in 2009.
"I found it hard to make new friends at first," the Form Three student says. "Part of the problem was I was conscious of being overweight. But my mum's friend is the wife of a kickboxing master in Foshan in Guangdong. So on Christmas Day, 2009, I decided to help myself and started to keep fit by doing kickboxing training."
Initially he made the tiring trip to Foshan to train every weekend. But since last year he has been training in Hong Kong.
"Losing weight was steady and continuous, rather than sudden," he says. "I lost it because of the training and taking more interest in what I was eating. Before I'd always order deep-fried snacks at food stalls. Now I know eating such foods is bad for your health.
"I'd also done weightlifting for a year in Primary Four. When I weighed 40kg I could lift 50kg. But I stopped because I wanted to spend more time on my studies."
His prowess at martial arts led him to demonstrate his skills in front of the whole school. Three schoolmates sat on a tree trunk, supported by two chairs. Although the trunk supported their weight without any problem, Cho-sing stunned the students by breaking it in two with a blow from his foot.
"Doing kickboxing and Sanda has boosted my confidence as a person and changed the way people regard me at school. Almost everyone knows me; the sport has helped me make friends and I also enjoy competing."
Muay Thai and Sanda are not too dissimilar, even though the sports have different origins, he says. "The key difference is the scoring method. Of course, the sports have different rules and taolu [forms]. But generally they are similar; that's why I was able to pick up Sanda quite quickly."
He is still in contact with the martial arts masters in Foshan. "They were amazed when they saw how fit I am now. My old trousers don't fit me now. My waist size before was 102cm; now I have a 76cm waist.
"Last time I put on my old trousers, they fell to the floor ... they were so big. It shows what hard work can achieve."