Boyfriend 33 Years Old Has Large Scale Rash, Flattish Of

Boyfriend 33 years old has large scale rash, flattish of varying size and not always round. welts in places. no blisters with liquid, not like poison ivy. head to knees, mostly on stomach and back. No knowledge of insect bites or exposure to someone sick. Started yesterday afternoon. No unusual foods except sushi on Sunday. Strangely, a 2 month old burn on arm is surrounded by the rash? He has never had chicken pox and his glands are not swollen or sore. I am not sure whether to be worried.