As Bartfull said, do get your daughter tested. An endoscope would be something to consider. They look for damage and abnormal cells by taking biopsies. Once you have a baseline on any damage, you can have follow up scopes to make sure he is healing if he is still having some issues. His blood tests should be checked periodicallly also to look for reduction of anti-bodies, again, to make sure his is healing and not getting any gluten. Another thing to consider is testing his thyroid function tested which is a simple blood test. Autoimmune diseases often come in multiples and Thyroid Disease is very common with Celiac. False Negatives for Celiac can be problematic, False Positives are rare.
There is a lot of information on Celiac Disease to digest. There is also a lot of crazy information about Celiac. Always make sure your sources of information are credible.