According to the 1860 census, there are dozens of men named John Kelly living in Georgia. However, none of them lived in either Brooks or Lowndes County.
Let me suggest that we assemble enough firm information about your friend's family to clearly identify him on the 1860 census. Then it becomes easier to locate his military records, and we're not guessing. We may have to take an educated guess at some point in the search, but you'd rather not start with a pure guess.