The mother had been dead for up to a week, WABC-TV, New York, reported. Police said the woman appeared to have died of natural causes with no signs of foul play in the apartment.
The boy told authorities he had repeatedly rubbed lotion on his mother's body in an attempt to heal her, the TV station said.
The apartment door was locked from the inside, but investigators said the child told them he couldn't open it and apparently had made no attempt to get anyone's attention. Neighbors told reporters they had heard no cries for help.
The boy was taken to a hospital to be treated for dehydration and malnutrition.
WCBS-TV, New York, reported authorities were alerted to the situation after a neighbor detected a foul odor coming from the apartment and a building maintenance crew found the woman's body when checking it out.
Daniel Zieser, the Union Township police director, told The (Newark) Star-Ledger the woman was found dead in bed.
Zieser described the boy, who is expected to fully recover physically, as a "spunky little kid."
"He's doing fine," Zieser said. "He made a statement to the officer, 'There's plenty of food in the refrigerator, but I just can't open it.'"
He said the boy's grandmother had lived in the apartment until last month when she suffered two broken hips. A neighbor told police the grandmother had tried unsuccessfully to reach her daughter in recent days, the newspaper said.