Has your son been near anyone that has measles? Does you give him enough water daily? There are many possibilities. To name a few: heat rash, allergy (to foods or elements), eczema, infection, scarlet fever, psoriasis, contact dermatitis. You don't mention age, but he could be allergic to whole milk (try skim for awhile), wheat products (toast all bread), oat, bran, honey, shell fish, peanut butter, soap (switch to unscented sensitive), lotions, detergent (switch to unscented sensitive), medicine, clothing (don't let him near flannel or wool, flea sensitivity if you have a pet (treat the pet and its bedding), etc. To find what is causing the rash, you must use a trial and error method. Once you discover the problem, avoid them and he will stop breaking out and will start feeling better.
I have allergies and eczema, avoid allergy triggers and take allergy shots; also see a dermatologist.