The blaze on Ardglen Drive left 18 units severely damaged or destroyed, according to Peel Regional Police. They say between 80 and 100 people have been left homeless.
Police say the boy's body was found inside a second-floor bedroom.
"He was having a sleepover visit," Peel Regional Police Const. Lily Fitzpatrick said. "He lived nearby, but not in that unit."
The first report of the fire came in at 3:19 a.m. ET, as an alarm activation, according to Michael Clarke, deputy fire chief of Brampton Fire and Emergency Services. The situation was upgraded after the fire department received phone calls confirming the fire.
When crews first arrived on scene, there were flames that came from the first and second level of the main unit, Clarke said. But because of the structure of the building, the fire spread rapidly in both directions from that unit.
“The crew attempted to make an aggressive rapid attack but they weren't able to make entry,” he said.
The exact number of units destroyed is unclear, Clarke said.
"But most of the units along that stretch have been destroyed," he said.
At one point there were 19 trucks and about 50 firefighters on the scene, Clarke added.
Fitzpatrick told CBC News that city buses are acting as temporary shelters for residents of the complex, which is near Kennedy Road and Queen Street.