Well, if he truly has celiac's maybe the gluten free diet was not *actually* gluten free, as CC may have been present. But I do believe that getting tested is way better and faster than trying and testing it for yourself!
Any doctor can write the orders for the test, and it's super simple! It's only a blood test :) ! The look for the proper antibodies.
Just make sure he DOES eat gluten in his diet before the test, otherwise, the antibodies may not show up.
It's very easy and you can get it done tomorrow :) if it comes back negative, at least you can rule this out! :)