John,
Did you have to visit the National Archives to access his records, or were you able to access on-line?
I do not find these records at Fold3 or Ancestry.com
George Martin
Register of Confederate Soldiers, Sailors, and Citizens who Died in Federal Prisons and Military Hospitals in the North, 1861-1865; NA Microfilm Publication M918. Records of the Office of the Quartermaster General, Record Group 92; National Archives, Washington, D.C.