Boston police identified a 22-year-old man Thursday who was fatally shot last week in the South End, allegedly by a 15-year-old high school freshman. Police identified the victim as Nicholas Martinez, 22, of Boston. The accused gunman, Raymond M. Concepcion of Mattapan and two other suspects pleaded not guilty Thursday to murder charges stemming from the slaying of Martinez the day before on Southampton Street. Police said Concepcion fired into a car in which Martinez sat while the vehicle was stopped in traffic near 85 Southampton St.Travis Andersen can be reached at email@example.com. Follow him on Twitter @TAGlobe.