Does he have a Bosniak passport? Does he say that he's Bosniak? What proof do you have that he's Bosniak.
Bosniak simply means Bosnian Muslim.
The Balkans is a very diverse region. In Bosnia, there are Bosniaks, Serbs, and Croatians. In Serbia, there are Bosniaks, Serbs, and Croatians. It's also the same in Croatia.
Adem Ljajic is a Bosniak, you can tell from his name.