Have him checked again. Despite how common it is, doctors sometimes have trouble diagnosing appendicitis. Did they do a blood test or a urine test to check for infection (white blood cell count), an abdominal ultrasound, an x-ray, or a ct scan? If not, ask for further testing to check for appendicitis. Some of the tests also may indicate if there is a Urinary Tract infection or a kidney stone. It also may be a digestive problem, since some of the symptoms are pointing that way.
If this doctor won't listen, take him to another one, to the ER, or a urgent care clinic.
I hope they find out what's wrong with him very soon.