there are chewing gums in serbia. in fact, most foods that are here exist there.
their winters are freezing cold, so gloves, hats, scarves. you can also send perfumes. the cosmetics there is very expensive.
little wallets you can pick up at marshalls or tjmaxx.
they love silver jewelry as gold is very cheap there. let me know if you need more ideas. i have been to that region about a million times.
men: i never go wrong with tshirts from gap (yes they know american brands and gap seems to be a hit). i always get size L for men and either do stripes or one color but has to have the logo visible.
kids: they're right now into ben10, so little plastic watches for kids. girls these lip glosses, or little barbie jewelry sets.
most people there smoke: so at duty free shop i pick up a carton or two of cigarettes, marlboro lights and then give a pack here and there when i see someone smoking. cigarettes there are very cheap but they're fakes, and taste awful, and they know cigarettes in the states are original (yes, i know smoking is bad but you can't change an entire country. they all smoke).
once he's there he can go to a local grocery store and pick up a bottle of lozovica or shlivovica (it's an alcoholic drink). every household carries these, and it's considered a good gift when going to someone's home. or whiskey if you want to keep it international. this is a country that fought everyone in the region, so their economy is bad, but they still try to live like they're used to in 90s.
does he know a word or two of greeting?if not pm me i'll help out.
no i am not from serbia but know a lot about the region.