What Causes Itchy Rash On The Buttocks?

My husband is on Ampicillin 500 mg. three times a day because he got salmonella food poisoning. This was verified through a stool culture. His upper body from his butt up is covered in a rash. He said it itches some but not majorly. He was given a prescription for prednisone. How soon should the rash go away after starting the steroid?

Thank you