It seems that your son has the Celiac disease
. The symptoms are suggestive for that.But, you have to be confirm that consulting with your doctor.
Your son should be on 'GLUTEN FREE
CASEIN FREE' diet.
As you are familiar with Celiac disease, I am not going with the diet composition details. But in addition to that, his diet should be free from milk products also (due to casein sensitivity). Let him drink plenty of water / fluids. He should eat fiber rich diet.
Celiac disease causes mal-absorption of nutrients from gut and results in malnutrition
with nutritional anemia
. That is why your son may look pale. Poorly digested foods creates flatulence
. Tummy pain is a symptom of all these.
As long you are not visiting doctor, put your son on 'GLUTEN FREE CASEIN FREE' diet, rich with dietary fibers, and plenty of water. If he is very anemic ( very pale looking skin and inner side of lower eyelid, easily exhausted, weak), visit doctor soon.