If we take all you wrote into consideration, especially the fact his lips were not helped by restrictive diets he tried so far, I would have not wasted a single day anymore, I would have jumped directly on FMT .
Why play war with small guns, when the largest gun is flushed down the toilet every day.
He is too young to experience huge improvement by liver flushing, that leaves just FMT and home made yogurt as the biggest guns to try.
It is not uncommon for young kids to be cured by just one single FMT treatment.
FMT is far less complicated than Liver Flush or restrictive diets.
See these videos:
Within 24 hours of fecal transplant, all symptoms were gone including bloody diarrhea up to 7 times day.
Consider fasting to be a diagnostic method.
If your son goes on Juice fasting or Water Fast for several days or maybe 2 weeks, his lips will heal if it was something in his diet causing the irritation.
People who are intollerant to milk, egs, wheat, gluten, cheese, potatoes, etc .... but they don't know what they are intollerant of, they can use fasting as the fastest method to find out if their everyday diet contains something causing thesymptoms, whatever symptoms they have.
Lips would usually heal within 7 days of fasting, if all you are intollerant of has been eliminated from the diet.
Important. I would start fast with a few days of juicing, before jumping on water fast.
I would flush my intestines (with magnesium sulfate, or magnesium citrate, or magnesium chloride or with saltwater flush or with enema or with colonics ) during the juice fast, so when I jump on water fast, my intestines would be 100% clean, no feces, no constipation.
It is common to be constipated, when started fasting, that is why intestinal flush is a must.
It is very important not to have anything inside your intestines, while you are on the juice or water fast, or you may get sick, hungry, even poisoned by the content of your own intestines.
That is the main reason why people give-up fasting. They know nothing about the flush.
For the best results, I would always sart a fast with liver flush, making sure my flush includes water with magnesium (sulfate or chloride or citrate ).
All common forms of inorganic magnesium tend to flush intestines (magnesium sulfate - Epsom Salt , magnesium citrate, magnesium chloride).
If his lips become natural after 7 - 10 days of fasting, that will be a proof of 1 or 2 things:
There was something in his diet irritating his intestines and/ or his microbiota is so imballanced that just keeping intestines clean and empty is enough to achieve cure.
If his lips don't improve within 7 - 10 days of fasting, it is a clear sign diet is not the main problem.
In this case the problem could be an allergy to something in his environment. It could be fungus in the house, it could be a single house plant or garedn plant, or it could be anything he is using as a soap or toothpaste.
It could be latex allergy.
It could be allergy to carpets, mattrasses, furniture, wallpaint, paints stored in the basement, etc.
It could be allergy to whatever you are using to wash clothes or dishes.
When it is not diet, the easiest way to find out if it is house, is to go camping.
2 weeks coamping in nature and the lips would improve, if he is alergic to somethng in the house.
No tent, no mattress. Sleeping og thick layer of dry grass, and tent made from brances. That way no contact with any chemical from any factory.
If I were you, I would have eliminated all and every cosmetics and soaps from the house. Put it all in one box , and store it in your garage, or anywhere far from the house for several weeks. including whatever you are using to wash dishes and clothes.
Dishes can be washed with hot water.
Clothes can be washed with borax.
For teeth: just dissolve unrefined Sea Salt in water and us the water for brushing teeth, washing teeth, swishing it.
Salt water can also be used for washing hands and body.
Also, borax can be used as main soap for hands, body and hair.
That way, anything coming from a factory, anything with fragrance, SLS, etc. has been eliminated, so it does not affect fasting.
That way, fasting is the most effetive tool to diagnose possible allergy or intolerance.
If he is not ready to start fasting today, you should make a homemade yogurt, and includ it in diet, if you ae positive he is able to tolerate yogurt.
As a yogurt starter you can use any probiotics or any other yogurt or sour cream.
If I were your son, I would take some homemade yogurt into a rectal syringe, and squeeze it into my rectum, and try to keep it there for a few hours.
It is not as effective as FMT, but for some people it works miracles, and you don't need a donor.
If I were you when your son was a baby, I would have taken some yogurt into a small glass dropper, and I would have squeeze it into his rectum, that would have stopped baby diarrhea, and would have helped his microbiota. Could have even prevented some of nowdays problems.