Yes, he is rather clueless over nonmilitary life.

[He seems to believe more in practical things and guidelines than in the concept of a god.] I think there is once a sect that follows this tenet, that is the Mu'tazilahs. But the Sunnis had absorbed them in, forming the basis of Islamic jurispudence and the Four Imams. This also reminds me of Alawites,who are very lax in their interpretation of Islam.

I defer to your judgement, since you watched the manga and read the anime. What's the main sect of Helmajistanis? Are they mostly Sunnis, in which case I can drop the idea for my fanfic easily? Or other sects? Could sectarian difference be the reason why Helmajistan be at civil war, or is it just KGB's plans?

Or let's just put this discussion to rest by assuming Sousuke is nonreligious, just as he's not attaching himself to any nationality.

It's part of his backstory, Sousuke Sagara was a Japanese stranded at Helmajistan, and was taught Islam by KGB and a faction that he was forced to join as a child soldier. So yeah, he will be considered a Muslim.

I want to explore him as a Muslim, since it will bring new challenges to the world. Sousuke is having feelings for Kaname, and if he thought that the Islamic Law applies to him now, he may have new problems to consider. How is he going to Salah when his charge is at risk? Can he fast and maintain energy? (there are pills for that, but I never tried it). Thus the requirement for late puberty.

Or it can go in other direction; Kaname discovers Sousuke is a Muslim, and when confronted Sousuke says 'yes', which doesn't fly with him willing to eat pork. Then a twist reveal is that Sousuke, and in extension Helmajistan, is a nation whose majority amount of Muslims are of a 'heretical' branch. No, don't look at me like that, there are many 'heretical' sects of Islam, with many weird interpretations.