POLICE are appealing for help to find the parents of a little boy found banging his head against a glass door in Tai Po government offices three weeks ago. The boy, thought to be about three or four years old, was diagnosed as suspected severely mentally retarded by a doctor after the police picked him up, said Inspector Josephine Yip Pui-lin. ''He can't even say his name and he has no facial expression,'' she said. ''It's very sad.'' He is described as being of Oriental appearance, probably Chinese, having short hair and a big nose, 91 centimetres tall and of thin build. The lad, barefoot but wearing a white Mickey Mouse T-shirt and blue cotton trousers, drew a curious crowd at about 10.30 am on March 28 when he began hitting his head in the corridor of the ground floor of the Ting Kok Road offices. A public appeal was ''a last resort'' after no one came forward to local appeals, said Inspector Yip. ''It's quite clear he's been abandoned,'' she said. The boy is now staying at the Chuk Yuen Children's Home in Kowloon awaiting a court protection order, the hearing for which is due next week. Anyone with any information on the boy or his next-of-kin should call the police on 660 3242.