A Charlestown man was found guilty Tuesday in Middlesex Superior Court of fatally shooting a Medford man and severely injuring the man’s father as he robbed the family’s Jerome Street home two years ago, said Middlesex District Attorney Marian Ryan’s office. A sentencing hearing is scheduled for 9 a.m. Wednesday for Gerald Sullivan, 39. Sullivan knocked on the house’s door with an accomplice on Feb. 18, 2011, and they pushed their way into the home when the door was answered, demanding jewelry and money, the district attorney’s office said. After a struggle, Sullian shot Johnny Hatch, 37, who died of his injuries, and his father, who is alive but still has bullet fragments in his head, the office said. Authorities identified Sullivan through DNA recovered at the scene, said the district attorney’s office.