As you probably know, the 10th Confederate Cavalry never served in Virginia. In 1864-65 soldiers of that command could have received mail in Georgia and the Carolinas.

The soldier in question received a discharge in October 1862 due to a claim of being under age eighteen. The discharge includes a physical description, information that you will not find elsewhere. Cox presumably remained at home until reenlisting due to reaching eligible age (October 1862 - August 1863). Whether he was actually being truthful or not is another issue.