He appears to be John Cumby of Co. "I", 42^nd Virginia Regiment. Details such as captured twice and joined U.S. service in the western territories fit the story. However, after reading his service record, if Cumby was a runner, he was running from Confederate service -- at least one court martial and six months AWOL after the first sentence was partially remitted. According to Cumby's statement made just after arriving at Point Lookout MD, he had been born in Virginia, age 28, had been conscripted (false) and wished to take the oath of allegiance to the U.S. Cumby was one of 155 soldiers and 9 civilians at Point Lookout who petitioned camp authorities to take the oath. After being released he volunteered for U.S. frontier service on June 13, 1864.

http://home.jam.rr.com/rjcourt52/cwprisons/lookoutn.htm
http://www.clements.umich.edu/Webguides/Schoff/NP/Point.html