Recurrent abdominal pain. Please advise.

My son has been suffering from recurrent abdominal pain around the navel for the last 1yr. Initially, it usually happens when he starts having food. No diarrhoea or vomiting is involved. His doctor diagnosed it as acidity and accordingly given Lanzol Jr, Mucaine Gel etc. It provided temporary relief but the pain returned after some months post treatment. A second dose was given but the pain returned again. Last deworming with Albendazole was done two-weeks back. Of late he has been complaining of the pain at night or any time of the day either with full or empty stomach. He also has the pain when he feels the tendency to defecate. The pain lasts for a few minutes and subsides after I massage his abdomen gently. He usually does not have a good appetite and is a fussy eater. His is current weight is 16.5kg and height is 103cm. Please advise.