Actually I have been worrying so much about your brother, and please remember besides what to eat, what NOT to eat is more important. According to my own case, GINGER, yes GINGER, even a little bit added just for cooking the food can make my mouth ulcer so bad, even a little bit ginger. So ask your mother NOT to use any GINGER, not even a little bit, for cooking for your brother. And when your brother eats outside, do not go to the 茶廳, western food (not fried ones) is better. Very very important , ask your mother not to use ginger.
One more thing: biscuits. All kinds of biscuits, including very healthy biscuits and even cookies. If your brother is so sensitive and gets mouth ulcer so easily, don't eat biscuits, as they are very "hot". Try to substitute bread or cakes for biscuits.
As your brother can't take Chinese medicine every day for the rest of his life, try not to eat food that easily causes mouth ulcer -- ginger, biscuits, nuts, and other "hot" food .