Vishal Kumar Nigam (30) was arrested at Allahabad in Uttar Pradesh yesterday, while the mother of the victim, Bharti Shinde (35), was nabbed on November 22, a day after the incident, they said.
Bharti, a widow, and Nigam lived together. They allegedly killed Aniket (Bharti's son) as they thought him to be an obstacle in their relationship, police said.
On November 21, Bharti thrashed the boy while Nigam held his hands and feet. When she took the boy to the hospital, doctors found his ribs were broken. Upon questioning, the crime came to light, they said.