Curious About A Red Rash That Has Broken Out All

Curious about a red rash that has broken out all over my 13 year old son's body. He is well rested, has had nothing unusual for lunch, and has used no internal/topical medication. Red rash (or hives) are really all over his body (arms, chest, belly, legs, etc). He is not hot to the touch and does not complain of major discomfort.