Police are seeking the driver who struck a 15-year-old boy riding his bicycle on Park Street in Stoughton at about 7:45 p.m. Saturday, said Executive Officer Robert Devine of the Stoughton police. The car that hit the boy was described as a light-colored, older model Cadillac de Ville that would probably have damage to the front passenger side. Stoughton Fire Lieutenant George O’Neil said the car apparently hit the cyclist from behind and kept going. A Fire Department ambulance took the boy to Boston Medical Center. Devine said the boy was expected to survive.