I’ve read up to page 23 and he says he believes in God but not in any specific religious dogma, also that his wife is a Catholic. What leads you to believe he is a Muslim, from reading elsewhere? If you are correct, it is likely an ethnic one, but not a practicing or believing one.
When someone from Bosnia refers to themslef as Bosnian it usually means they are Muslim. Bosnian Serbs and Bosnian Croats generally refer to themselves as Serb or Croat.
There are many mixed marriages in Bosnia, had this person mentioned the town it would be easier to verify.
It’s a good bet this was Mostar, and Muslim is a good bet because of that. “The Siege of Mostar” was brutal, most especially on the “Muslim” half.