Police have registered a case against a man on charges of sexually assaulting a 12-year-old boy, who is his relative, in Maharashtra's Thane city, an official said on Wednesday.

Last month, a teacher called the parents of the Class 6 student and informed them that there was a change in the boy's behaviour and he had been misbehaving with other children and getting irritated frequently, an official from Kasarwadavali police station said.