Going along with what keyzersoce said... it sounds like it could be thrush (infection with the yeast Candida). You should be able to scrape off the patches with a little effort. If they do scrape off, the mouth might look swollen or bleed slightly, which would also signify thrush. Your child might be prescribed an antifungal medicine. Look up info on candidiasis for more info and to see if it sounds like what your child has.
Our newborn had similar symptoms and was diagnosed with "thrush," an infection caused by the yeast: Candida albicans. However, we were told thrush is commomly found in newborns, infants and the elderly. Only a doctor can tell you if your nine-year old son has it. Good luck.
It could be canker sores or something associated with a bacterial or viral infection. If it does not clear up in 7 to 10 days then have a dentist see him. If there is no definitive diagnosis take him to an oral surgeon.