Police say Matthew Zhang's father dropped the boy off at his mother's residence on Mississauga Valley Boulevard at 10 a.m. Sunday. When the father contacted Matthew's mother later that afternoon, she refused to return him, according to police.
Police believe the boy's mother, who they've identified as Wei Chen, has fled the area with her son.
Police said Chen doesn't have custody of the boy, and isn't able to take proper care of him.
She drives a 2005, grey four-door Honda Accord, with Ontario licence plate AVAM 009, according to police.
She is described as Asian, five feet six inches tall, with a thin build and collar-length black hair.
Police say Zhang, who is also Asian, is five feet tall with a thin build and short black hair.
When last seen, the boy was wearing a blue or green winter coat, a sweater with a hooded sweatshirt, and grey runners with a green stripe.