S.H.
Is it itchy? Are they flat red spots, or are they swollen or raised?
If it's itchy, has swelling or raised spots, then it's probably hives. He may be having an allergic reaction to something. Could be something he is inhaling. I've had an allergy or something for the last 20 years that causes hives for several days. Still do not know what causes it!
Does it always appear in the same places? Or does it move around? Mine moves around.
Have you changed (or has the manufacturer changed...) your laundry detergent, fabric softener, etc? Body wash? Shampoo?
If you can get him to an allergist while he has the rash/hives, they may be able to help you more.