It's a wonderful image but unfortunately contains few clues. Was your ancestor or his brother an officer? A single-breasted frock like this would be appropriate for a lieutenant or captain. The garment is a regulation 9-button frock coat common to the infantry and cut pretty much correctly for the war period; however, the inverted V of the cuff piping doesn't contrast with the material of the sleeve the way it should if it was the infantry light blue. (Officers likely wouldn't have piping like this but in volunteer units it's at least possible.) The shoulder straps - if that's what they are - are bizarre, as are the collar "decorations" which seem to be aping the "false buttonholes" seen on cavalry and artillery enlisted men's shell jackets. The apparent Colt M.1849 Pocket Revolver is likely a photographer's prop as that was a civilian model and though they were carried by officers he doesn't have a holster for it. The belt and its buckle are correct for officers.