It's possible to have both (I do). The symptoms of celiac can vary widely, but GI and malabsorption symptoms can mimic Crohn's. However, a GI would be able to differentiate through testing. There are different indicators for both in the blood work. Additionally, scopes w/ biopsies can pinpoint crohn's damage (ulcers, inflammation, etc via colonoscopy) vs. celiac damage (flattened villi via endoscopy).
If your son suspects Celiac, it's important that he continues to include gluten in in his diet through testing to get an accurate diagnosis. The "treatment" for celiac is maintaining a gluten free diet. Once someone starts a gluten free diet, damage done by celiac begins to disappear and testing is not accurate (it can take anywhere from a few weeks to a few years to undo the damage depending on how severe the damage is).
It's probably not the answer you wanted to hear, but I hope it helps.