It may be that your son bit his tongue or has a localized reaction to something he ate?
The medical concern would be that this might be the start of an allergic reaction, so any further swelling, cough, trouble breathing, wheezing, means an immediate trip to the ED.
If the pain stays localized then Anbesol and sucking on an ice cube for short periods are the best options.
Your son may also be coming down with a virus that causes mouth sores, as these are fairly common.
Hope this helps and your son feels better soon.
Louise Sivak, M.D.