"As a child he didn't seem to understand our instructions, and I couldn't understand what he was trying to say," says his mother, Cheung Yu Wai-ting. She saw a delay in his language development and "odd" behaviour when he was young.
After Tsz-king's diagnosis, his mother was desperate to find ways to improve his condition. After attending many workshops and feeling lost, something positive finally happened.
"A teacher at his school told me he was good at drawing and that I should take him for lessons," Cheung says.
This idea changed Tsz-king's life. After art therapy classes, his world opened up to others. "At the start, he didn't talk to his teacher - he just kept drawing," Cheung says. "Later he spoke and even listened to her suggestions."
Tsz-king, sitting beside his mother, suddenly says: "I feel happy ..."
He leaves it to his mother to finish his sentence, "... when I draw".
And he really can draw. Two of his pictures - My Favourite Festival (Dragon Boat), and Unique Hong Kong (Tsing Ma Bridge) - won the Hong Kong Autism Awareness Alliance's "Autistic/Artistic" competitions in 2011 and 2012.
William Fan Tak-wing, a psychiatrist and president of the alliance, says art therapy helps people with speech and communication problems, like autism. He says: "Drawing helps them to relax and communicate their ideas, feelings and experiences. Through art, others understand them and offer support. Parents can see their strength and appreciate them more."
There is no official figure for the number of autistic children in Hong Kong. But Fan says it is widely accepted that one in every 100 children is autistic.
He says art therapy needs professional training, but a good art teacher, or artist, can use the medium to help autistic children and others with special needs. It's vital to "accept them and not be judgmental - and emphasise sharing and communication", he adds.
Graffiti artist Cheung Sai-lung, or Pantone C, has run workshops to help children communicate for more than a decade. "Many of my students aren't good communicators; they didn't want to talk when they first came," he says. "But art can engage people, offer them a chance to express their inner feelings. Graffiti has no set rules or boundaries. It's about respect - teenagers need that today."
Tsz-king met Pantone C at the artist's Art Therapy-themed exhibition, SpongeBob x Graffiti, at Park Central, in Tseung Kwan O. "Tsz-king's young, but his work shows he's already developed his own style," Pantone C says. "I believe he'll be a great artist one day."
Tsz-king's mother says: "I want my son to be happy and have a hobby he enjoys. One day, when I'm not here, his love of drawing will keep him company, so he won't be too lonely."
Pantone C's exhibition runs until September 1.