The records of the 21st Arkansas are missing much of their role during the Vicksburg campaign. I suspect John was wounded and/or captured early in the campaign and was not with the regiment at the surrender of the Vicksburg garrison where it appears they were rather depleted and commanded by only a Captain, Capt. A. Tyler whose parole document appears in the records. The unit suffered 37 casualties at Port Gibson early May and later had men captured at the battle of the Big Black River. Not all of these men's records include POW records.

21st Regiment, Arkansas Infantry

21st Infantry Regiment was organized during the winter of 1861-1862 by consolidating four companies of J. S. McCarver's 14th and six companies of G. W. Lomoyne's 17th Arkansas Battalions. The regiment was involved in the Battles of Corinth and Hatchie Bridge, and reported 27 killed, 41 wounded, and 58 missing. It was then assigned to General M. E. Green's Brigade, Department of Mississippi and East Louisiana. The 21st sustained 37 casualties at Port Gibson and was captured at Vicksburg on July 4, 1863. It was not reorganized after the exchange. Colonels Jordan E. Cravens and William G. Matheny, and Majors W. M. Dowdle and Harrison Moore were in command.
http://www.civilwar.nps.gov/cwss/regiments.cfm