I’ve often wondered about this myself. I use Sensodyne toothpaste as I have very sensitive teeth from gum recession. I don’t think it’s the toothpaste but I could be wrong. I used to get the dreaded mouth ulcers before being diagnosed but not a single one since going on the diet. It is possible your son is reacting to something else in his diet. It is not unusual to have issues with milk proteins / lactose for a while until the gut properly heals itself. Many people also have issues with with Corn. What age is your son?