I'm Writing Concerning My Dad, Age 88. He Recently Had

I'm writing concerning my dad, age 88. He recently had a scan, an endoscopy and a colonoscopy. All test were normal but he is still having same symptoms. Nausea, weak and no appetite. Today the office of his gastro Dr called and said he had some irritation that might be a food allergy and to immediately start a gluten free diet. I know celiac is a serious condition but my concerns are this seems to be a "fad" and is it really possible for him to develope this at his age?