My Son Is 7, He Has Been Complaining Of Stomach

My son is 7, he has been complaining of stomach pain for some time now. He was diagnosed with chronic constipation and taking miralx for about 4 years now. Lately he complains more and more. We tried avoiding dairy and had him tested for celiac disease. He then became pale, and was lathargic and sleeping alot. He was taken to immediate care where they found: problems with creatin levels, bun, and acid in his blood. They say he is not absorbing nutrients and fluids. Please tell me what you think may be the cause of this. His white blood cells are also a little elevated.