With the range and units it does not make any sense for that test to be for Tissue transglutaminase IgA.  That normal range would be very logical for total IgA.  Please ask your GP when you see him/her.

Just repeating what my gastroenterologist was saying. Sorry - not a helpful post of mine, as I wasn't even sure what he meant!

That’s interesting thank you. I’ll definitely be going back to the GP on Monday and I’ll update

I agree with @trents--Oats naturally contain a protein called avenin, which is similar to the gluten proteins found in wheat, barley, and rye. While avenin is generally considered safe for most people with celiac disease, some individuals, around 5-10% of celiacs, may also have sensitivity to avenin, leading to symptoms similar to gluten exposure. Your son may fall into this category, and eliminating them is the best way to figure this out. Some people substitute gluten-free quinoa flakes for oats if they want a hot cereal substitute. If you are interested in summaries of scientific publications on the topic of oats and celiac disease, we have an entire category dedicated to it which is here: https://www.celiac.com/celiac-disease/oats-and-celiac-disease-are-they-gluten-free/   

Welcome to the forum. I just want to mention that B9 is a vitamin where too much can be toxic: Vitamin B9 (Folate): The UL for Folic Acid is set at 1,000 mcg (1 mg) per day for adults. This limit primarily applies to synthetic folic acid found in supplements and fortified foods, not naturally occurring folate in food. High intake of folic acid can mask the symptoms of Vitamin B12 deficiency, which can lead to neurological damage if left untreated. This is because folic acid supplementation can correct anemia caused by B12 deficiency without addressing the underlying neurological damage. Some studies suggest that excessive folic acid intake might increase the risk of certain cancers, such as colorectal cancer, particularly in individuals who have precancerous lesions.