There are many causes of chronic abdominal pain in children ranging from mild to serious causes. The commonest cause is constipation. Others are many ranging from peptic ulcer disease to food allergies e.t.c.
So you need to see a paediatrician for an appropriate evaluation

Who prescribed the drugs you gave? Hope it's not self-medication? For the stooling, mix ORS and be giving him and watch. If the stool becomes too frequent, bloody or any fever, please take him to the hospital. And if he is also not tolerating the ORS, take him to hospital