Hello,
Due to the suddenness of appearance of rash along with no prior history of any rash,it can be due to a viral infection or an allergic reaction.Scarlet fever(strep infection with a rash) and other viral infections and allergic reaction due to contact dermatitis have to be ruled out.Apply calamine lotion on the rash and consult a pediatrician soon.Treatment for all is different and before prescribing any medicine in children ,it is important to confirm a diagnosis first.
Hope it helps.Take care and pls do keep me posted.
My 3 year old got a rash on her face, neck, and upper chest and back. It was light colored but splochy red. There were a lot of tiny bumps - the doctor described them as "sandpaper-y". We gave her benadryl and it didn't do anything for her. we took her to the emergency room where they did a cbc and strep test. (Keeping in mind that my kids are not in school / daycare. I stay at home with them every day, and nobody in the house has been sick.) My daughter's strep test came back very positive - the doc explained that the rash was Scarlatina - a bacterial problem caused by the strep. She hasn't complained of sore throat, no cough, very little fever (nothing above 100* F). The only issues with her were the rash - slight itching, and just seeming tired. Take your son to the doctor.