I recommend reading Dee Brown's "The Galvanized Yankees" (University of Nebraska Press, 1963).

If you have Aaron Newton's Compiled Military Service Records for his service in the 32nd Tennessee Infantry, it should include his Federal POW records which will tell you when and where he took the Oath of Allegiance and enlisted in Federal service. From that point, you can pick up the story of the U. S. Volunteers on the western frontier in Dee Brown's book. You should also be able to find his U. S. Compiled Military Service Record in the appropriate Federal CMSR.