@Hitsnooze
DS2 was like that. Approximately at the same age he was diagnosed with wheat, egg, dairy, tomatoes and citrus allergy. We were also recommended to avoid poultry.
I am happy to tell you he will happily eat those now at 14, except for the chicken he doesn't like particularly.
His reactions and manifestations were similar, extremely thick hives, rash, and later on angioedema. He is also allergic to penicillin and sadly this one is still on (with cats, and dogs).
At the age of 12 he developed asthma, also linked to the same immunes system reaction.
All these allergies were caused by excessive use of antibiotics and reflux medication in early months of life. He had sever reflux, causing arena and leaking of vomit in ears and lungs causing infections. He received tons of antibiotics and was on omeprazole for reflux and it is now knows that both antibiotics and antireflux alter gut bacteria and the immense system.
Because of my son's long list of allergies, I ended up doing and baking everything from scratch. I would buy spelt flour for the bread and cakes, do risottos instead of pasta and so on.
For breakfast, you can offer rolled oats or do your own bread.
Recently new studies have been published about the impact of ultra-processed food, so avoid industrial food, such as crisps, puffs or any food that use food parts instead of a whole (pea or rice starch, extract, powder, flour, ...) , or food that have a long list of additives. At one points, my son was very allergic to the bright and shiny red colouring.
The best advice I can give you is not to use any ingredients that didn't exist in 1920 . So fresh and whole food. Single ingredient food. Low processing food such as a bottle of passata which is tomatoes and a pinch of salt. Stay clear of the sugar while you are at it.
I did a lot of soups and he still likes them at 14. He will snack on soup after swimming training.
At the age of 3, after 2 years of draconian diet, he was able to eat normally again with our ill effects.
I didn't cook one meal for him and one meal for everyone else, we all had the same no wheat/egg/dairy.... dinners and lunch. Only breakfast was slightly different because sometimes the kids would have a croissant (we are French) .
It is not that hard, it can be done and there is so much more choice now in supermarket than there was 14 years ago.