I'm having trouble tracking down the proper period he served in, his uniform seems an odd mix of different eras- ie., the tall-collared coat of the 1872 regs, with the dark aiguillete of about 1900 (when the coat style had been changed twice) and the 1911ish style collar insignia arrangement.
Closest I can find is this one below from about the S-A War period, but the collar is quite different. It would be so much easier if he was wearing his headgear, as that can sometime nail it down to within a year or two!

Its got a collar like this one (ca. 1872) but the insignia and aiguillete is different.
