I would say he is a Union soldier. He looks to be wearing a frock or roundabout jacket with a sack coat over it. And his waist belt is standard federal issue as well. Did Confederates use all those items? Yes they did but its not likely they would have all of those items put together like this soldier does.
I would venture an educated guess that this soldier was in winter quarters and decided to have his picture taken. We can assume he is wearing a heavy civilian shirt, frock/roundabout and sack coat because it is so cold. The saber and knife could be his or just as easily photographer's props.