Unlicenced rickshaw driver Chen Chuanliu told the Chinese wire service Xinhua News he chained his son Lao Lu to the post while he picked up customers because he feared kidnappers would steal the boy.
Chen's four year-old daughter was stolen earlier last month. His wife was unable to care for Lao Lu because she was "mentally disabled."
"I have to work to support my family," the 42-year-old told Xinhua News.
"I don't even have a picture of [my daughter] to use for a missing person and ... I cannot lose my son as well."
Shoppers saw the boy outside the Huaguan Shopping Mall near Liangxiang, 20km southwest of Beijing, before contacting authorities. Chen was ordered to remove the shackles.
The boy cannot be taken to a local nursery because the family is from another province and therefore does not qualify for state help.
Chen, who earns just $8.10 a day, said he could not afford childcare. He has also refused "many good offers" to put Lao Lu up for adoption.
"To chain him is better than losing him," Mr Chen said.
5 weeks ago