Abdomen Pain Very Badly..

My younger brother, aged 12 yr, is feeling pain in abdomen occasionaly since 2-3 yrs. Symptoms are :- Hard pain in abdomen(in intestine) suddenly for 2-3 hours and then he feel relaxed completaly. At that time he feel vometting and bitterness in throat and also feel that bad smell and bitter water comes from his stomuch. His toilet is normal but at such time, when he go to toilet, hardly come (Patli) toilet and urine pass yellowish. We consulted a Gastro surgion doctor and he suggest us for Ultrasound. The report shows : "Found some enlargement in Illiac fossa". Dr. gave us time for 10 days to take some medicine but still he feeling pain sometimes hardly, then just to reach hospital, he feel relaxed completaly. Is this pain may be appendix / Ulcer? What test is good to know the disease? Sir he is in class 5th and this sudden pain is very dangerous in school and when he is alone.