The accused, Manoj (28), had gone missing after the incident, which took place more than a month ago. He was arrested yesterday, DCP (West) Pushpendra Kumar said today.
According to police, Manoj was pressuring the child's mother to leave him with her first husband. On March 10 afternoon, when the child was alone at home, Manoj allegedly attacked him with the Gandasa axe.
The woman, who works in a factory nearby, happened to return home at that hour and saw Manoj leaving the room with the blood-stained weapon. She rushed to her house and found her son lying in a pool of blood, with his jaws split apart.
A special team was sent to Manoj's village in Shahjahanpur district of Uttar Pradesh and raids were conducted, but he could not be tracked down. He was finally arrested from Delhi yesterday, police said.