I can understand your brothers reluctance to join any group. I didn't join here until 3 years after my first cancer diagnosis. Even after, I rarely, if ever, posted during any of my subsequent treatments, but after I did. Anyway, It doesn't matter where he is getting proper advise from, wether through you here or his medical team. Dr. Google is very beneficial in my hands, others, maybe not.
As far as Mucocitis, milk, milk products, have lactic acid, sugar, so it can burn. I couldn't drink or eat it, and luckily I had a Peg tube when needed. A straw may help get past some of the burning, sores. Many develop thrush and is difficult to distinguish Mucocitis from thrush other than the removable white coating on the tongue, which is not always necessarily present or it can be a viral or bacterial infection too or combination. If so, that needs to be treated properly with the appropriate medication.
My radiation center sprayed out my mouth daily with saline solution after each radiation session, which helped with thrush, and Mucocitis. A water pic spray may be similar if the center doesn't do it. They also did IV hydration.
The salted warm water, baking soda mix is pretty standard. As mentioned, if the salt burns, cut it out. I seen a study where virgin organic coconut oil was added to this mixture, and reduced mucocitis pain. More studies are needed, but said there is no harm trying. I also used Glutamine powder to rinse, swallow. This helps repair tissue in the mouth, throat, stomach, in addition, is often used for neuropathy. Others have used Manuka honey for mouth healing.
My last treatment I only used NeutrSal, Supersaturated Calcium Phosphate Rinse, instead if the salt/baking soda rinse. It helps with mucocitis and dry mouth, but I don't know if it's available in your country. MuGuard here, may also be similar.
Brushing the teeth may be difficult. Some may use a baby brush, a sponge stick which helps get in the back of the mouth, and a neutral, non irritating fluoride toothpaste. I always used Biotene toothpaste, no alcohol mouthwash.
Good luck, and hope this helps.
