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It is difficult to diagnose without proper details and clinical examination.
If he is having pain on left side or middle not associated withfever or loose motion
then it could be due to gas and acidity.You can try to give an antacid
syrup.If on right side or radiating to back or chest it is best to take him to a doctor .Try to make him relax in lying down position .Give him adequate water.hope I could help.thanks