The police said that the body of Soumendranath Bal was found in a sack, dumped at a secluded place near Bethuadahari station.
Family sources said that Soumendranath went missing on Tuesday morning from a place near their home.
He went out to play with other children in a field near rail station, family members said.
Soumendranath's uncle Jagannath Bal said, "The abductors called me over telephone on Wednesday several times. They demanded Rs 2.5 lakh from us."
Shisir Saha, circle inspector of Nakashipara police station, said, "The culprits are being traced from their mobile numbers from which they called the victim's uncle to demand money.