Found this in Lindsley's Annals,
"On the 17th of September, 1861, the railroad was repaired; after which the battalion marched to Bowling Green, Ky., with Capt. Woodward's independent company of cavalry, Brown's brigade of infantry going on the cars. These were the first troops at Bowling Green, and constituted the advance of Maj.-gen. Buckner’s division. Two detachments of fifty men each were made from the battalion, the one sent forward to Munfordsville, near the crossing of the Louisville and Nashville railroad at Green River, under command of Lieut. Jas. H. Lewis; the other, under command of Capt. J. B. Biffle, sent to the vicinity of Hopkinsville, Ky., to look after Federal troops, and especially what were then called "Home Guards" in the State of Kentucky on the Federal side of the struggle. The Biffle detachment met the enemy near Hopkinsville in one or two engagements. Geo. W. Barham, private of Company E, from Perry County, was killed, and young Montague, of Company D, Wayne County, was shot, the ball passing through his lungs. Barham was the first man killed in the army then commanded by Buckner—afterward by Gen. Albert Sidney Johnston. At least it was so understood at the time."
Lieut. Col. James H. Lewis, Lewisburg, Tenn

Looks like your ancestor holds a special distinction in the AOT, being the first soldier of that army killed in battle. At that time the unit was still officially the 2nd Battalion, later they became the 1st/6th Regiment, more commonly known as Wheeler's Regiment.

Regards,
Paul Maloney