What Causes Persistent Fever, Headache And Sore Throat?

hi, have a 5 year old white boy with high fever ( 105deg) for 5 days, headache (occasional), sore throat, non-productive cough, cracked, bleeding lips. now, fever gone but more perioral rash lesions and petechiae of hard palate. throat cx negative, wbc 5.7, platelets 269,000, blood cx pending, cxr neg, sinus xray neg. no lesions on hands or feet. any thoughts?? says he is feeling better but still not normal activity level (irritability persists). our dr thinks sinus infection. on zithromax since yesterday.