The effect is different among individuals. He may have bowel problems, stomach pains or vomiting, but then again he may not. Some coeliacs do not manifest any outward symptoms, but the damage is being done on the inside. He would not be absorbing his nutrients properly and could end up suffering from deficiencies, even malnutrition. Stomach cancer is also a real possibility. Coeliac Disease is the easiest of all diseases to treat; no pills, injections or any kind of medical intervention - just diet. Sure, it's a pain to have a restricted diet, but if the Coeliac makes the effort to do the sensible thing, they will feel so much better. It is well worth it.