Take him to the dr. to get him diagnosed. Could be a couple things..... First thing that comes to mind is 5th disease (one of the rash/illnesses that a child can get)- it's a pretty benign condition, can be a bit itchy but the biggest concern is exposure to a pregnant person. Many times, it's on the trunk of the body, arms or it appears on the face - it's also known as "slapped cheek syndrome or disease" because of the ruddy appearance that it has on the cheeks. This rash can occur after a small bout of illness. It can also give the appearance of looking kind of lacy. Here are some pictures.
http://www.lib.uiowa.edu/Hardin/md/dermnet/fifthdisease56...
Another thing to consider, does your son have or had a sore throat, stomach ache, low grade fever - have you ruled out Strep, when untreated can lead to Scarlet fever, which produces a rash?
Also, food allergy or exposure to something may give him a rash and benadryal could help.
It's always best to contact you dr. and start working backwards to think of any other symptoms or exposure to anything that he may have had to cause this because the dr. will want this information. If you don't go to the dr., at least see your local Pharmacist because they may have dealt with this in the neighborhood and may be able to give you the advise that you need to treat the rash. They are a wealth of knowledge! Good luck.