The collision happened in Eastgate just after 3pm on Wednesday (December 7), opposite the Eastgate car park and Lincolnshire Co-op Funeral Services.
According to eyewitnesses, the injured man had parked his car at the side of the road and was in the process of helping his child out of the car.
It was then that a passing vehicle clipped the door of the parked car, causing minor damage to the door and minor injuries to the man who was knocked by the door. The offending vehicle then drove away from the scene without stopping.
A Lincolnshire Police spokesman confirmed the incident, and said that a standard collision investigation is ongoing.
• If you witnessed the incident or have any information call Lincolnshire Police on 101, quoting incident 292 of December 7.