Bad rash

Patient: Our 8 yr old boy 4 days ago complained of an itchy ankle, within 4 hours of that we found a rash virtually all over his body. It initially looked like an allergic reaction to something, environmental, dietary. We brought him to a walk in clinic. They agreed it was allergic and prescribed hydrocortisone cream and amoxicillin because of a slight ear infection. The following day the rash was much worse. We discontinued the amoxicillin and brought him to emerg. The physician quickly assessed him as allergic and prescribed prednizone and benedryl. Day 4, rash has gone away completely and then 18 hrs later returned full force. The rash is itchy, no other symptoms ie no fever or malaise. What should we do?