Serbian POWs in German Camps

Hello, I am helping a friend who is trying to find out what happened to her great uncle who disappeared around the end of WWII.

He was Serbian and his name was Vicentije Vicovac. He fought with the Yugoslav Partisans under Tito and was about 19 or 20 when the war broke out.

The family were told he was imprisoned in a POW camp in Germany during the war and released towards the end of 1944.

Is there a list of POW camps where Serb soldiers were imprisoned and does anyone know if there are any online indexes available to search for POW's in these camps?

Sorry for such vague information - I am a WWI buff myself and really know very little about events in Eastern Europe during WWII.

Regards,
Elle