The boy, Lau Chun-yin, sneaked into the bedroom and used a pre-programmed computer function to call his father, the South China Morning Post reported Thursday, quoting from a court hearing Wednesday.
The boy's father alerted police, who quickly arrested Ma Ka-lam, 17.
The boy's mother, Leung Sau-king, told reporters that Chun-yin had been working with computers since he was 3 years old.
"I wouldn't say he's a genius, but Chun-yin learns relatively fast," the Post quoted her as saying.
Ma, who took $35 from Leung's wallet, pleaded guilty in the July 21 robbery and was ordered to serve 200 hours of community service, the newspaper said.