What Causes Skin Rashes On Legs After Taking Flu Vaccination?

my son had a live flu vaccine and was given a steroid 6 day dosepak for a crackling in his ear the same day. That evening after the vaccine and two doses from the steroid dosepak he had a rash on his legs. I stopped the medicine until I talked to the doc. I talked to the doc and the pharmacist and they both said to go ahead and resume the medicine. I did. No more red rash appeared until tonight. Now it is really bad on his upper thigh and buttocks. should we go to the emergency room or wait to talk to the doc tomorrow. He feels no itching or pain but there are many many deep red bumps.