The boy, Yamato Tanooka, went missing Mt Komagatake in Nanae on Japan's main northern island of Hokkaido, which is inhabited by wild bears.
Yamato, his older sister, mother and father came to a park near the forest on Saturday, but the parents became angry when the boy threw stones at cars and people, Japanese police said.
On the way back home, they made Yamato get out of the car and left him alone in the forest, driving the car about 500 meters away, TV Asahi and other reports said.
"They said they went back to the site after a few minutes, but the boy was no longer there," a local police spokesman earlier told AFP.
About 180 rescuers and police officers widened the search area on Monday, mobilising sniffer dogs and horses to go deeper into the woodlands, according to broadcaster NTV.
Footage showed scores of officers clawing through overgrown forest and tall bushes as a helicopter hovered overhead.
Police searched in the dark Sunday night with torches in hand and calling out for the boy, who was wearing a black jacket, navy blue pants and red sneakers when he disappeared, according Kyodo news agency.