The unfortunate reality is that he is indeed unknown. This is not out of the ordinary, as photographs such as these are often given to archives or other institutions with little context to them. This should function as a reminder to always make sure to mark your photographs (with an archival pen!) with a name at the very least. Your photographs might end up in an archive one day and you might very well be "unknown man in front of backdrop" for the rest of time.

With that said, the photograph you linked is actually a cropped version of the original photograph. This is the original photograph, as it appears in the collections of Library of Congress. It's a recent acquisition, having entered the collection in 2017 through a gift from an individual who in turn seemingly purchased it from a collectables dealer.

The Library of Congress dates the image as being from around 1918 to 1919, which on first glance seems like a good conclusion. He wears the fabric insignia of the United States First Army on his shoulder, which came into being in August 1918. The insignia wasn't approved until November that same year. The insignia disc on his collar shows crossed rifles, which identifies him as belonging to the infantry. There's an F beneath the crossed rifles, which means that he belonged to Company F. Unfortunately, there's no sign of regimental numbers anywhere, which makes it impossible to confirm that this man was indeed part of the 369th Regiment (The Harlem Hellfighters). Furthermore, I can't make out what appears to be a ribbon on his chest to the left (if that is what it is, seeing as there appears to be some sort of fastener on it). However, to the right, there is what appears to be an expert rifleman badge. Unfortunately, beyond that, there is not much else we can deduce. He wears a campaign hat, which was typical of American soldiers of the era. Perhaps this was taken in the United States or France after the conclusion of the First World War, considering the man's qualification badges and fine uniform. This might push the date of the photograph a little bit, to perhaps 1919 or even 1920, if he might have remained in Germany on occupation duty.

Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!