If you put canker sores or mouth ulcers into your search engine you will find a lot of information about them. Name is Minor Aphthous ulcers..and can be caused by several things, SLS, sodium Lauryl Sulphate can be the culprit you can get toothpastes that are free of it, he could be allergic to varoius foods including Cereals Milk, Fruits like lemons and Oranges, chocolate. His diet may be deficient in B1 B2 B6 B12 and C ..It s also states there can be a hormonal factor...he is 12 and there could be a genetic disposition. I know that teens seem to be extra suceptable, he could try dabbing ordinary table salt on it a few times a day and there are all kinds of preperations to dab on that will soothe and heal. Google for more and how to help him..Good Luck