The incident happened just after 7:30 p.m. ET at the family's home in Sainte-Martine, located in the Montérégie region.
Quebec provincial police said the boy slipped into the pool after he was left unattended for a few moments.
Police say there was no safety fence around the pool.
The boy was taken a local hospital and then transferred to Montreal Children's Hospital.
The incident happened a day after a five-year-old girl disappeared in the Rivière du Lièvre in the Laurentians.
The girl's body was found Saturday morning, three metres from where she had fallen in while playing with an older girl on the river's edge.
Drownings up in Quebec
In total, 33 people have drowned in Quebec this year, 11 more than at the same time last year.
Raynald Hawkins of Quebec's Life Saving Society says the warm weather only adds to the concern since more people are out trying to cool off in pools, lakes and rivers.
Drowning can occur in as little as 10 seconds, so it's important to always be cognizant anytime you or your children are near the water, he said.
"Please follow the safety tips like wearing your life jacket if you're going on the boat. . . and if the [child] isn't within an arm's length, they are too far," he said.
"If we follow only those two recommendations, we're going to see our numbers going down."