Correctly Diagnosing

My 11 year old son, who recently had tonsils removed, has had another case of high fever with throat blisters.  A strep culture was negative and a second culture was sent to an outside lab, we don't have these results yet.  His only symptoms are high fever, sore throat and blisters.  No nausea or diarreah (diarrhea).  He can't eat and barely drinks.  What could possibly be wrong with ruling out tonsilitis and strep.  This sickness occurs at least every 3 or 4 months.  His last round was in May and tonsils were removed in June.  Please leave suggestions.