Suggest Medication For Abdominal Pain And Burning Sensation During Urination

my 12 yr old son has been having frequent pains in his side and upper abdomen area for months (about 5 or 6) it comes and goes every few weeks he also is having burning when he urinates so the doctors have checked for a UTI and kidney infection....everything is normal he even had a renial ultrasound and a MRI of his pelvic area and abdomen area and he is still having problems. the last month he has been feeling fine but last night the pain started up again.....we are at a loss as to what is going on.