The matter came to light on yesterday, when the victim's mother made complaint at Krishna Nagar police station in this connection, according to police.
The accused is still absconding.
The victim, a student of Class 1 lives along with his parents and six sisters at their residence in Kanti Nagar area.
"Boy's mother works as maid to earn her livelihood. Last month, she along with his son, went to work at the house of a businessman, identified as Pawan Bansal in Krishna Nagar.
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After the assault, the victim started crying and narrated the incident to his mother.
Vivek, along with his landlord's son Rajesh and one Kallu threatened the woman of dire consequences if she approached police.
The mother, afraid from them, remained silent. However, when her son's condition deteriorated on 31 July, she rushed him to a nearby government hospital, where doctors confirmed sexual assault.
A case has been registered and efforts are being made to arrest the accused, police said.