The boy was missing since October 20 and a case of kidnapping was registered by the police. Police have detained one person, a neighbour, who is suspected to have abducted the boy, said a police official.
The sack containing the body was spotted by locals at Jwalapuri locality today, who called up the police.
The sack was also stuffed with gravels, raising suspicion that the murderer wanted to ensure that the sack containing the body doesn't float in the drain, said the official.
The boy lost his father more than a year ago and lived with his mother, being her only child.
The body was sent for a postmortem examination and later handed over to the relatives. The cause of death is yet to be ascertained, officials said.