Thomas Horan also put himself to bed before he was discovered by police when they broke into the flat in Wythenshawe, Greater Manchester.
His mother Sue, 27, suffered from epilepsy and was thought to have suffered a fit as she took a bath.
The alarm was raised by a neighbour and friend Corina Livingstone, 32, when the child answered her telephone call on Sunday evening. He told her his mother was asleep in the bath and he could not wake her.
"I went over to the flat to try to get Thomas to open the door but he could not find the keys," she said. "I knew something was wrong so I ran home to call the police, telling Thomas to be a good boy."
Greater Manchester Police said officers discovered Miss Horan's body lying under cold water. It is thought Thomas had slept in her bed, eaten a bowl of Weetabix and played with toys for several hours before he was found.
Miss Horan's mother, Dorothy Gregson, from Hyde, Greater Manchester, said: "We believe Sue had a fit and drowned in the bath. She would usually have a bath just after teatime which she did as normal on Sunday."
Thomas is now being cared for by relatives.Reuse content