It's encouraging that everybody so far has pointed at the Balkans, which matches the conclusion I'd come to - he was probably a Serbian partisan.
I was googling images for various WWI British mounted units (I've posted another part of this photo as a separate topic -
Can anybody throw any light on this unidentified mounted (WWI?) soldier) when I noticed a photo including a chap in a vaguely similar shirt, hat and waistcoat
(I've attached a crop of that photo for comparison). The photo's the fifth one down on this page -
http://www.levantineheritage.com/salonicamap.htm with the title "Some of the Comitadjis who worked for the British in the Struma Valley". So I googled comitadji (anti-Ottoman partisans in the Balkans) and Struma Valley (the Struma is a river in Bulgaria and Greece) and found a photo from the Library Of Congress (with no known restrictions on publication) of
Serbian comitadji prisoners, from which I've also attached a crop for comparison.
I've also attached the chap from the original photo. I think that's fairly conclusive, don't you ?