Units typically did not complete organization when they failed to meet the prerequisite number of troops to have in the unit to allow it to be called a company, battery, regiment, etc.
These late organized regiment were predominately made up of Georgia "State Troops" and were under the command and control of the Georgia State Governor Joseph E. Brown. When the Conscription Act became law, states were required to produce a certain amount of regiments. One way he met this quota was taking organizations already formed for coastal defense, reorganize them, and give them to the Confederate Government. I suspect your ancestor was in one of these state organizations.
I would do four things:
1. Look at the State organizations that were from where your ancestor lived at. These records will not be on Fold3 because those are only records of units turned over to the Confederate Government. They will be at the Georgia State Library and Archives in Morrow, Georgia.
2. When a organization failed to complete organization, the men were not told to go home and have a nice war. They got sucked up into another organization. I would check where the 58th Georgia was being organized and what other regiments were being formed at the same time and look in those regiments for your ancestor potentially going there. The 57th and the 59th Georgia Infantry Regiments were organized at or near Macon, Georgia (I forget what camp) on or about May 8, 1862. The 57th Georgia had the 2nd Georgia State Troops (not to be confused with the 2nd Georgia State Line) and the 59th Georgia had the 7th Georgia State Troops as their base organizations. These regiments were then augmented with conscripts and volunteers to fill them to Table of Organization strength. Because they were formed the way they were, they were not allowed to be called "volunteers." For example, the 39th Georgia Volunteer Infantry Regiment.
3. Look for the records of UCV Camp 675 and see what organization your ancestor joined under.
4. See if he filed for a pension and see what organization(s) he said he served with in the pension application.
Please let me know if you have any questions.
Gerald D. Hodge, Jr.
Lieutenant Colonel, U.S. Army (Retired)
M.A. Military History - Civil War Concentration
Research - Preservation
Historian: 39th Georgia Volunteer Infantry Regiment