He appears to be a Prussian or, if this photo is post 1870 (which is almost certain to be true) a German Garde du Corps; which generally had red collars and lapels. He is in highly ceremonial and traditonal wear, as evidenced by the Order of the Black Eagle adorning the red overcoat on his chest.

Compare it to the man on the far right.

The eagle adorning his helmet, the red overcoat, the R for Rex, all suggests he is in fact a member of this regiment. This would mean he is in the personal cavalry unit of the Prussian, and later German king. The Sabre and Riding boots only further confirms this.

Great photo, thank you for sharing! Are you able to date it?