Suggest Treatment For Rashes On Lower Legs And Knees

hello my son is 4 years and I do not know what the problem is because since last Friday has a rash on the lower legs, knees, ass and elbows. Before the appearance of the rash for 3 days in the evenings if vomited after the appearance of the rash vomiting disappeared, he felt well today but in the evening throwing up again there is no fever.
I was a doctor, said that, but it is a rash of nursery please help