Chief Executive John Lee on Sunday urged the family of a mainland boy abandoned in Hong Kong to come back for him, saying such things can't be tolerated in the caring society he is striving to build.

The boy, 12, was found wandering around Kwong Wah Hospital on Friday with police classifying the incident as child abuse and neglect.

Officers have got in touch with the boy's mother who travelled with him to Hong Kong by express rail from Jiangxi province, RTHK understands. The woman is believed to have since returned to the mainland, but is said to be willing to come back to fetch her son, who was born in Hong Kong.

Lee said a protection order for the boy has been sought, adding that the courts and the Social Welfare Department are experienced in handling matters regarding the care of children.

"I believe they will handle the welfare of this child properly and take care of his health both physically and mentally. But I think this really shouldn't have happened," Lee told reporters at the airport before a flight to Singapore.

"A key aspect of my governance is to build a caring and inclusive society, so we really can't allow something like this to happen."

The CE said that police are investigating whether anyone is legally liable for the boy being abandoned.

He added that the incident has left Hongkongers feeling both sad and uncomfortable.