9 yr old severe stomache aches Doctor thinks acid-reflux?

My son has stomach pain accompanied by naseau every morning, sometimes with vomiting and diarreha. Dr. prescribed ranitidine for 14 days twice a day and then only as needed. Well it has been 20 days and if I don\'t give him two doses each day he is right back to the same symptoms, and frankly I am getting a little scared. Should he be taking this medication that often, for that long? What could possibly be causing these pains? Doctor thinks it is acid-reflux happening during the night, so we are giving the medicine an hour before bedtime and right away when he wakes up. While taking the medicine he seems fine and back to normal, but everytime we quite he is sick again. HELP!