I wonder what makes you guess that.
Makings on a collar were different for officers, That would mean he was an officer. On the other hand there is no/or is not visible/ indication of his rank on his hat. (there should be two, on a shoulder board/each arm/ and one on a hat) I just don't remember if that ( insignia on a hat)was the requirement for the time period.
You see that photograph is not of the highest quality.
So, IF I'm right about the collar, IF we disregard his hat. Plus there is something barley visible on his shoulder board, It could be a single star (an indication of a rank) or it could be a number of his regiment. So many IFs.
On the other hand if we disregard his collar markings (maybe I don't remember correctly which is which)- he was a private..
Hence I said it was a guess.