My Nine Year Old Son Had Tonsillectomy And Adnoidectomy Surgery

My nine year old son had tonsillectomy and adnoidectomy surgery 9 days ago and four days after surgery he developed a rash on his back and stomach that does itch a little. He seen his doctor for follow up two days ago he said the rash was a viral infection but he has now had the rash for six days and it's getting worse not better. He is currently not taking any medication so it can't be an allergic reaction to meds. What can it be?