I took my daughter to my Naturopath and she diagnosed her when my daughter was 3. My Naturopath also informed me that a usual MD would have said she is NOT celiac because she is borderline. In fact, NONE of the pediatricians in the allopathic community bothered to look at food allergies as a possible cause for all of her symptoms. In the Naturopathic community they treat these allergies much more seriously than MDs do. She is every bit and more seriously ill with this disease than any MD has admitted to. The month I took her off wheat, her mind started to function and she started learning really fast. She had gotten to the point she wouldn't talk any more (at 3). My Naturopath said that my daughter can NEVER have gluten. This proved to be true, when at age 7 she had seizures due to eating wheat.
I read the book "Gluten Free Girl" and realized that getting wheat completely out of the diet is very difficult, due to the problem of cross-contamination. She cannot use the toaster for her gluten-free bread, for example. My daughter actually has learning delays due to this disease, because it wasn't diagnosed until she was 3. Your son may also only be experiencing an intolerance to wheat and not full-blown celiac disease.
Again, the best people for diagnosing this from my experience, are Naturopathic physicians because they have a better handle on how much of the offending item your child can take.
The symptoms can vary greatly - everything from stomach pains, to lethargy, to incredibly impacted bowels from not being able to "go", all the way to seizures, and autism if left untreated.