Suggest Remedy For Allergic Rashes On Head & Toes

my son (19 months, 23 lbs) gets a terrible rash from head to toe (almost could be mistaken for measles if you didn't know any better...) anyhow, he was tested for allergies- all negative- however he continues to get the rash whenever he eats dairy products (and it takes weeks for the rash to disappear)- his other symptom is frequent stools (not loose) and then the rash when eating dairy help!