Acting on a complaint by Childline workers, police rescued the boy, Monish Thomas, from his house in Anchal in nearby Kollam district and admitted him to a government hospital here, police said.
Quoting the Childline workers and neighbours, they said his father Thomas Francis (35), an auto driver by profession, and his second wife had caused burn injuries.
The father was arrested later.
Investigation was on and the stepmother would also be taken into custody soon, police said.