What Causes Large Oval Rash On Buttocks Spreading To Legs And Stomach?

my 5 year old has a somewhat large oval rash on his shin. It started out looking like a bite but spread out in a red rash - middle is now dry but the ointment from the dr. and oral antibiotic don't seem to be making a difference. He now has raised spots similar to how this one started on his legs and a few on his stomach area. What might this be????