He's also an apple that did not fall far from the tree. His father, Tso Shu-yan, and elder brother, Ralf Tso Sing-yeung, who is eight years older than Rex, are seven-time and four-time Hong Kong champions, respectively. Rex himself started boxing at age 12 at South China Athletic Association.
"At first I was only hanging around and didn't treat the sport seriously," recalls Rex, who lives in Tuen Mun. "In secondary school, I would just lie on the sofa and play video games all day after school, unless I had boxing practice. I did not have a purpose."
But then he began participating in some overseas tournaments. "I fought against some foreign boxers and they were much better than I. I realised I had to do more to compete at an international level," says Rex, who completed his studies at Lui Cheung Kwong Lutheran Evening College in 2007.
He worked as a warehouse assistant for a year before becoming an assistant coach at his club. He joined his current club DEF Boxing in Sheung Wan six years ago. There, he underwent a transformation, becoming a top local fighter.
Rex's coach, Jay Lau, saw he had potential, though he didn't think much about becoming a professional. But he had run out of local opponents.
"There is always only one or two, at most four, rivals in my weight category in local competitions organised by the Boxing Association. Other categories also face the same problem. I rarely had a chance to try something new as the opponents were all too familiar with each other," Rex explains.
He won the title at the year-end Boxing Championships in 2008 and 2009, which pits the two top boxers against each other. The winner is crowned Hong Kong champion. In 2010, he was automatically awarded the title as there was no opponent in his category. "I felt sad I couldn't even have a fight in the biggest annual tournament. The incident inspired me to improve and look abroad."
So he turned pro. "I want to promote boxing in Hong Kong," he adds. "I hope I can be an ambassador for the sport."
Last summer he went for a month to the Philippines to train there. He sparred with some local champions of the sport. He realised he could stand his own against more experienced fighters.
Professional fights have more rounds - four or more three-minute bouts - than amateur ones. "In the Philippines, I focused on my defence to prepare against the more powerful punches of professional opponents. My coach reminded me to keep my head well protected every time I finished my attacking combos," says Rex.
He now trains six days a week and runs 10km every day. Last September, he won his first professional title. After five pro wins, he took on his biggest challenge so far - a battle for the vacant UBO Asia Pacific Super Flyweight Title last month. Rex managed to knock out his opponent, Thai boxer Torsak Singabasak, halfway into the first round of an eight-round bout. It was the first-ever professional boxing title won by a Hong Kong citizen.
He added another belt to his collection at a Singapore invitational tournament earlier this month.
He has now set his sights on yet more glory. "I need more victories on my way to an international boxing career," Rex says. "I haven't got a plan yet but I guess I will target different Asia Pacific titles first."