http://www.healthline.com/adamcontent/erythema-multiforme?utm_medium=ask&utm_source=smart&utm_campaign=article&utm_term=Erythema+multiforme&ask_return=Erythema+Multiforme goes into depth on all forms of this disorder/condition. No I never had this sort of allergic reaction, instead I got anaphalaxis reaction to a bee sting - to the point of not being able to breath because my throat swelled shut. Discovering your allergic or sensitive to something is usually a 'painful' trial and error thing, fortunately modern medicine can right most problems associated with allergic reactions. Granted your son is uncomfortable (at the very least) at the moment, but once the doctors get it treated you and your son will know that antibiotics are a 'bad thing'. Since your son is allergic to antibiotics I would suggest that you speak to your doctor to having tests ran to see if he is allergic to anything else - especially in the medicine department. Especially sulfonamides, penicillins, barbiturates, and phenytoin (dilantin). since your son reacted to one he might react this way to others. Further many people who have one allergy end up with others as well, its best to discover what they are through tests than through trial and error.