If your cousin has hepatomegaly (It is not a disease it just denotes that his liver is enlarged)you might have to check his blood picture and reticulocyte count to rule out any hemolytic disease of the new born.Serum lipid profile was advised by your doctor to rule out any lipid profile.Liver enzymes were advised to rule out any defect to the function of the liver due to deposition of waste products in the liver.Serum TSH and cortisol were done to assess his metabolism.Regarding his stomach pain you can give him syrup carmicide 5 ml if needed.Regarding his diet you should avoid very hot or very cold foods.Avoid fat containing meals like fried foods and spicy food.Give him a lot of fruits and make him drink enough water.The treatment of the condition will depend upon the blood results