Welcome to Just Answer and thanks for your question. Since this isn't getting better, your son needs to see a doctor for a second opinion. A family doctor would be the one to see, since the emergency room is mainly for stabilizing patients with serious illnesses. They docs there are not as familiar with treating skin infections and rashes. He probably needs another antibiotic if the skin infection hasn't cleared, but the comments about blisters make me wonder if this was a viral illness to begin with that caused the rash, and a bacterial infection occurred afterward in the scratched lesions. In any event, he needs to see another doctor.