I know this is a little expensive, if I remember right it was 200 or 300 dollars. Enterolab is what I had done and you can purchase just certain parts of the test. It is a stool test and cheek swab (genetic test). The stool test gives you other info. You could look and enterolab.com and see if they offer just the genetic test. If you do yourself first and find out that you have 2 positive genes then you don't have to test your child because you know they get one of those two genes from you. I have a celiac gene and a gluten sensitive gene, so I know all of my kids without doubt shoud NOT eat gluten. Unfortunately, we found out too late and they are addicted. I would get him on a gluten free diet ASAP. It will be much easier now than it will be as a teenager. Good luck, if you need anyhelp with that website let me know.
I know there are other good sites for genetic testings too, but that is the only one I had experience with.