I might be way off on this.
I'm assuming this Boykin lived around Troup County? If so I looked and could only find one Captain Boykin. He wasn't a James Boykin but a John Thomas Boykin, Jr, born 3 May 1835 and died 25 June 1901. He was the only child of John Thomas Boykin, Sr.(1794-1869).
I went to this web site which seems to have the "BOYKIN FAMILY PAPERS" and even lists a picture of Captain John Thomas Boykin, Jr in his uniform.
http://www.trouparchives.org/man/ms015.htm
It says he was a Captain of Company F, 21st Georgia Infantry Regiment, C.S.A. So I looked up his records and it says this:
Boykin, John Thomas – Captain - July 9, 1861. Resigned, disability, May 31, 1862.
So he resigned in 1862. That would mean he had to be in a Georgia militia unit later on. So I find the Milita Unit in Troup County.
It says that Troup County was in the 37TH Military District. But it was more commonly refered to as the:
Georgia Militia Regiment of Troup and Heard Counties
Col. T. C. Miller
I didn't find a Captain J.T. Boykin in the companies of this militia Regiment but found a "independant" militia cavalry unit that was loosely associated with the 37TH Military District Militia. It was in this unit I found a private J. T. Boykin Jr. It could be that they still referred to him as a Capt. out of respect.
Boddie’s Company, Georgia Cavalry, Local Defense Troops, “Troup Independent Cavalry”Troup County
Capt. C. M. Heard
Capt. T. A. Boddie
1ST Lieut. S. F. Beasley
2ND Lieut. James N. or H. Fannin
3RD Lieut. N. G. Swanson
1ST Sgt. D. A. Walker
Sgt. J. P. Cline
Sgt. J. W. Hodnett
Sgt. A. A. Lane
Sgt. L. T. Swanson
Corp. G. V. Boddie
Corp. J. E. Davis
Corp. J. S. Herring
Corp. F. P. Hudspeth
Privates:
J. W. Akers, S. T. Beasley, J. M. Birdsong, J. T. Boykin Jr., J. H. Burks, A. F. Chislom, R. L. Christopher, W. H. Clark, T. H. Cox, J. H. Fomby, J. H. Glanton, W. E. Glanton, M. H. Hart, W. P. Herring, Robert Hill, F. Holle, M. F. Hurst, Godfred Kener, W. J. Matthews, J. H. McFarland, T. S. McFarland, J. M. Ponder, E. A. Reed, R. H. Richards, T. H. Robertson, William H. Russell, L. E. Scott, C. B. Sharman, J. W. Story, J. W. Strong Jr., J. J. Walker, J. G. Whitfield, E. M. Wilkinson, W. H. Wilkinson, L. C. Wisdom