This Celiac.com FAQ on celiac disease will guide you to all of the basic information you will need to know about the disease, its diagnosis, testing methods, a gluten-free diet, etc.
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What are the major symptoms of celiac disease?
Celiac Disease Symptoms
What testing is available for celiac disease? - list blood tests, endo with biopsy, genetic test and enterolab (not diagnostic)
Celiac Disease Screening
Interpretation of Celiac Disease Blood Test Results
Can I be tested even though I am eating gluten free?
How long must gluten be taken for the serological tests to be meaningful?
The Gluten-Free Diet 101 - A Beginner's Guide to Going Gluten-Free
Is celiac inherited? Should my children be tested?
Ten Facts About Celiac Disease Genetic Testing
Is there a link between celiac and other autoimmune diseases?
Celiac Disease Research: Associated Diseases and Disorders
Is there a list of gluten foods to avoid?
Unsafe Gluten-Free Food List (Unsafe Ingredients)
Is there a list of gluten free foods?
Safe Gluten-Free Food List (Safe Ingredients)
Gluten-Free Alcoholic Beverages
Distilled Spirits (Grain Alcohols) and Vinegar: Are they Gluten-Free?
Where does gluten hide?
Additional Things to Beware of to Maintain a 100% Gluten-Free Diet
What if my doctor won't listen to me?
An Open Letter to Skeptical Health Care Practitioners
If it were my child, I'd keep him off of gluten and dairy as recommended by the dietician, especially since he's not well and this is possibly celiac. It won't hurt him to be off of gluten and he may even improve, but if this is celiac disease or gluten intolerance, he will not improve on the gluten. Right now your goal is for him to be well as soon as possible. I personally would wait until your son is healthy and recovered to even consider reintroducing gluten for a challenge.
There is nothing worse then not having clear answers. I would encourage you to be persistent. Eight weeks is not typical for a "bug." Follow your gut, if you are getting the cold shoulder see another doctor/doctors. Get and keep copies of all test results as part of your son's history to help new consults avoid retesting and to help determine the next steps in his care.
Have you seen an allergist...?
Thanks for the feedback everyone. I've been gluten free for just over a year and have been pretty good about staying away from processed foods and bread substitutes. I was just really jonesing for my grandma's banana pudding. Zanthan gum, soy and corn don't bother. So Googling the other two things I ate around the same time I only came up with one tiny web hit pointing to the fig of all things but I'm doubtful. It was an article about foods high in FODMAPs (whatever those are) which included figs and prunes. It suggests they trigger gastrointestinal symptoms. Well yeah! But I eat prunes all the time so I feel like I'm just reaching. Adding this to my food log but I may take a chance and try another piece of cookie one more time in another day or so and cross my fingers before ruling them out completely. I don't believe any company is perfectly infallible even if they do have a great reputation. I'm still super sensitive and have been occasionally triggered by other products that others seem to have no problem with.