It looks like an 1885 pattern US Army uniform, except for the fact the coat has two rows of buttons. The spiked helmet also matches the US Army pattern and has a US late on the front. From the absence of any rank markings on the sleeves or shoulders, I would assume he was a private. I've attached a couple of pictures to compare it to. I would guess he was regular army, except for the double row of buttons which was usually reserved for officers.