A 15-year-old Esko High School football player died Friday after collapsing on the field during a game. Jackson Pfister collapsed just after 9 p.m. during a varsity game at Veterans Field in Aitkin, Minn. He was taken by ambulance to Riverwood Healthcare Center with what police described as a serious medical condition. The boy, who would have turned 16 next week, died shortly afterward. Jackson was the son of Brooke and Matthew Pfister of Esko. His mother’s Facebook page indicates that Jackson had a congenital heart defect and has undergone open heart surgery. Aitkin police and the Ramsey County Medical Examiner’s Office are investigating.