He won't play for Croatia and Serbia. In fact if someone suggests that he will play for Serbia is either completely ignorant or stupid. Don't have any idea about Turkey but I don't think that there is anything on it.
Anyway, his parents are Albanians from Kosovo. The problem isn't that straightforward to choose between the place where he was born and the native country because Kosovo isn't accepted in FIFA. Despite more than 90% of Kosovans are Albanians, still there are a few differences between them and the fact that they have been seperated for more than a century means that the culture and many other things are very different. For example the first time I went to Albania was when I was 16-17 years old, a few years after the Kosovo war. For more than a half of century it was forbidden for Kosovans to go from Yugoslavia in Albania and vice versa. So there are a few differences which makes things even more difficult. Although the most of Kosovans choose to play for Albania there are a few cases where they ahve opted for different countries (Valon Behrami for example). In other cases the Albanian FA has been that much corrupted that have asked for money to certain players in order to play for them so naturally they chose other countries (Granit Xhaka being the best example). And in some bizarre cases they haven't even made an offer at all (surprisingly Xherdan Shaqiri never received an offer to play for Albania despite he was willing to do so). Many of them have declared that if Kosovo gets accepted in FIFA/UEFA, they will play for Kosovo. (Shaqiri for example celebrated UCL victory with a flag of Kosovo while Xhaka when asked to sing a song in National team, he chose to sign the Albanian anthem).
I think that if Kosovo was in FIFA it would have been an easy choice for Adnan, but as it is things are much more difficult. It is entirely possible that he hasn't ever been to Albania. We'll see what happens, but I would be okay with him choosing whatever nation he choose. Probably will boo him if Albania ever plays Belgium and he is on their shirt though.