Now that I have seen the photo, his uniform does not appear to be Confederate. Did a quick search of the NPS Soldiers & Sailors System and found only one man that fits the description: John L. Cook, Principal Musician, Co. G 21st Mass. Inf. That would be a Drum Major, I believe. His name could be John S. and the "L" is a transcription error or visa versa. I would search both names and include "Cooke"s.

The question now is how did your ancestor come by the photo. Knowing where he served would help. Give us his name, home county, rank, company, regiment if known. Was he ever in the Army of Northern Virginia with Lee? If not, did he ever serve as a prison guard? The 21st Mass. was part of the Army of the Potomac and served mainly in the Eastern Campaigns.