As someone who was born into a Muslim family, and someone who lived in Muslim countries (Saudi Arabia and Pakistan) for the first 17 years of my life, I have to say that I have not come across any term such as a “Soldier for Allah” in the Muslim world that is used in the same concept as “Soldier for Christ”.
It could be that the t-shirt is suggesting what Morbyd suggested “that he's a soldier for Allah in the same way that some Christians call themselves soldiers for Christ”.
However, anyone who has lived in the ME or S. Asia would say that the first impression one would get when you hear the term “Soldier for Allah” is a Jihadist who would use force to get the point across.
Either way, the bottom line is that from a situational awareness standpoint it is not wise to BLATANTLY FLAUNT anything that would provoke negative reaction. I am not saying what the kid should or should not wear, or should or should not believe; it is a free country after all. However, one needs to be aware that if one is going to do something like that then they should be prepared for some looks, maybe some alienation and quite a few questions defending what you are promoting.
Heck, many of y’all know that I take our Second Amendment (Right to Bear Arms) to heart, and I am not afraid to admit it. However, I am also fully aware that when I state my stance to an audience where most have not been exposed to firearms in a positive manner, or all they have heard from the time they could comprehend is negativity about firearms, I will get some looks, maybe some alienation and quite a few questions defending my stance. I am also aware to not to go to a night club wearing a Heckler and Koch t-shirt with a picture of an MP-5 saying “In a world of compromise, some don’t”.
In some ways it is similar to a sales representative going to meet the maintenance guys at a Power Plant in an AMG SL55 wearing a Submariner. There is nothing wrong with it, but you need to be aware that you might not be walking out of the plant with a PO.