I would certainly believe he served as a guard, but in Fannin's not Symon's reserves.
George Martin
Private William Mattox, age 47, enlisted in Co. B, 1st (Fannin's) Georgia Reserves, April 25, 1864 at Atlanta, died October 25, 1864 [no further records]
See Ghosts and Shadows of Andersonville starting on page 92 @
http://books.google.com/books?id=GXKQM93jOPwC&pg=PA92&lpg=PA92&dq=1st+Georgia+Reserves+Fannins&source=web&ots=Wstdn4XweK&sig=dJIs1KKjraLXqBWQTGyn5Os3ttI&hl=en&sa=X&oi=book_result&resnum=8&ct=result#PPA92,M1