This horn player shall remain anonymous as the Real Photo Post Card or RPPC, was never mailed and has no identification written on it. The photo card stock dates to 1908-10 based on the kind of graphic logo for the stamp box on the back. This proves to be a very useful way to date these kinds of photos and it would likely make this gentleman an American since the box says "one cent stamp here". But after that, who knows where he is from?
It is an unusual closeup pose with a focus on the instrument. The horn is unusual because it has 4 valves but it is still a single horn in F. The upper thumb valve is for transposing into Eb. This is an uncommon style horn and may have been made in Germany. It has an unusual oval patch on the bell which might be a makers mark, though it seems far too large for that.
What I like most about the photo are the gentleman's medals. I've tried to find extra details in a magnified scan but there just aren't enough pixels to see anything clearly. The bright one - 2nd from right, has a lyre motif similar to the awards I got in high school band, so my guess is that they are medals for music performance. Based on his apparent age and the time period, it seems unlikely they are medals from the 1899 Spanish-American war. It's possible that the photo was intended for publication in some band instrument newletter as part of a testimonial letter.