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That sounds like a serious reaction to something. He might want to consider a trip to the ER. Usually a rash from an allergy to something isn't painful. Shingles rashes are painful. While they are usually confined to one side of the body (from mid-back around the side to the navel in front), it is possble to have a generalized rash. That requires antiviral medication to decrease the severity and duration of the symptoms.
If it is a bad allergy, he needs to watch out for trouble breathing or swallowing. Those are serious signs and he would need the ER ASAP.