It happened at a house on Los Altos around 5:40 Saturday evening.
Investigators say the child and his 21-year-old mother were watching television in a back bedroom of the home when the mother fell asleep.
When the woman woke up she discovered the child was missing. She looked around the house for the boy and found him floating in the pool.
Investigators say the door from the house to the backyard had been left open.