Another person chiming in that yeah I would recommend allergy testing too. My son-in-law has major eating issues due to allergies. They started in college for him and problems come out as his throat getting mucous-y and closing up, heartburn and other GI symptoms. It hasn't been life threatening, other than the poor man can't eat hardly anything. He keeps a live document that lists his current "can't eat" and updates/changes it constantly. He did the whole allergy testing thing and was getting allergy shots for a couple of years after they moved to OH and was doing really well - he able to add a lot of things that made cooking and eating out easier back to his diet (like dairy, rice and chicken eggs). Then their health insurance changed and the doc he was seeing wasn't available to him anymore. So, as people do, he just stopped them. He has slowly been getting worse again - things he had been able to eat for a good while suddenly not agreeing again. It finally dawned on him that maybe the allergy shots were helping a lot more than he realized. Hopefully he can find a new allergist and get started again. He does have a GI doc too.
It is always an adventure when they are here for a visit trying to figure out what we/he can eat.
One of my girls turns out has a gluten sensitivity. After multiple docs having no clue what was wrong and why she was so nauseous all of the time in college, I just decided to do all of our cooking for the week she was home for Thanksgiving one year gluten free to see if it helped her. It did. She felt so much better and it made her realize that she had been having trouble with gluten for years and just didn't know it. She can eat gluten and doesn't have to totally avoid it, but she does feel better when she reduces it. Gluten free products are so much more available in the mainstream market than they used to be. You have to read labels and watch things carefully still though.
The absolute best cookbooks I found/have for tasty gluten free recipes are the Americas Test Kitchen ones. They are always super involved and can take extra time to do things the way they want, but they are always good. A lot of the time I think it is too much, but the GF eating isn't as necessary health-wise in my kitchen usually. One of our now favorite recipes is their Mexican meatloaf that uses ground corn tortillas for the binding. Their lasagna is really excellent as well.
There are two ATK GF cookbooks (Vol 1 & 2) - here is a link to a version that has both in one book. We tended to like more of the recipes in Vol 2.
My SIL finds most of this recipes online- he uses a lot of really specialty type ingredients at this point as his diet is so eclectic.