Hey Cass, sorry to hear about your DS, but try & see it as a positive that you have a diagnosis now & with a change in diet you should see a massive improvement!
Our besties little girl was diagnosed celiac at 3, had so many issues with tiredness, behaviour & was such a tiny, skinny delicate girl. Since changing her diet, the improvement is nearly miraculous! She is soooo much healthier & happier now. As for diet, there are some fantastic alternatives out there nowadays for gluten free options & he will learn what is safe for him to eat & to ask if he's not sure.
Hugs for your family on this journey xx