Wanted to follow up on Anne-Marie F.'s response. Your son could also have severe gluten intolerance and not necessarily Celiac disease. I have severe gluten intolerance, except I have the opposite problem: constipation. I also have casein dairy allergy and severe soy intolerance. I didn't discover my food allergies and intolerance until I was 40 years old. Wish I would have known sooner, because I suspect I've been sick with at least one of the three my entire life.
Your Pediatrician or the gastroenterologist will only test for Celiac disease, because according to allopathic medicine, ONLY Celiac disease is a disorder that they can treat. If you want to know if your son has gluten intolerance--or, indeed, any other food problems--you most likely will need to take him to see a Naturopathic Doctor and have him tested. That's what I ended up having to do when the gastroenterologist told me I didn't have Celiac disease. Well, good news. But I still have severe gluten intolerance and need to be on a gluten-free diet.
Another problem with mucousal problems is a problem with being able to digest fats. Again, a Naturopathic Doctor can test for that and a whole host of other problems as well.
You're being a good mom and looking into all of this. Your son will thank you. The best gift you can give your son is to find out his allergies/intolerances so that his body and immune system aren't adversely affected for life. I know. It's not easy getting the body built back up after one has been so sick. If I'd only known when I was younger I had these problems with food, I could have deleted the problem food area from my diet. Probably wouldn't have been so sickly growing up either.
Good luck. If you want to chat or want the name of the Naturopathic Doctor I use in Madison, WI, contact me.