Anne, I found a Henry D. Combs in the 2nd Regiment, US Volunteer Infantry, and since the 2nd and 3rd Regiments, US Volunteer Infantry consisted of Confederate prisoners from Rock Island this is probably your ancestor. I have done some research in the Official Records of the War of the Rebellion on the 3rd Regiment, but not much on the 2nd. I do know that the 3rd Regiment was posted up and down the Overland Route from Fort Kearney, Nebraska Territory to Julesburg, Colorado Territory, and also at Fort Laramie. Both the 2nd and 3rd were utilized for frontier service, which was guaranteed in the soldiers' enlistment "contracts". A good book to read is "Galvanized Yankees" by Dee Brown if you can find it. Below is a little history of both regiments that I got from the OR.

February 20th, 1865, the Second and Third Regiments, US Volunteers begin to leave Rock Island for Fort Leavenworth, KS ( the 2nd and 3rd Regiments came from Rock Island Prison)

MILWAUKEE, WIS., February 20, 1865.
(Received 12.50 p.m.)
General DODGE:
Two regiments from Rock Island begin to leave for Leavenworth
to-day by companies; two companies leave to-day. Instruct commanding officer at Leavenworth to arm and dispose of them. Will leave for Saint Louis to-morrow; wish to see you there.
JNO. POPE,
Major General.