Rash on 2 year old

Samantha

My son is coughing and now has a rash. He also got a new chair at my parents house that he sat in all day. I don't know if this is allergic reaction from chair or a viral rash. What do you guys think? We are on day 3 and its everywhere. should I call the doctor at this point. I have been using hydrocortisone, aveeno oatmeal bath, and benadryl and still not better.