Hello
Sorry to know about the condition of your son.
Abdominal pain with slight distension,appetite loss,failure to gain weight in children can be due inflammatory bowel disease/hepatitis/intestinal parasites.
If I were your treating doctor ,I recommend,
-Taking the child to paediatrician.His physical examination and certain investigations like blood haemogram,USG abdo,
liver function test,stool analysis will be required.
Colonoscopy may be required for diagnosing inflammatory bowel disease.Further management depends upon results of investigations.
-Give him small frequent meals as he has appetite loss.
-Maintain hydration by plenty intake of water.
Give semi soft cooked and fresh food to eat.
-De worming therapy can be given by Tab
Albendazole 400 mg one dose.
Hope this information helps.
Wish him good health