5 year old son woke up with hives all over his body (arms, abdomen, back, buttocks, legs, feet, hands, ears, neck, face) yesterday (monday) morning. i?

Hives: Is he itchy or febrile? Is the rash persistent or it comes & goes, any changes in his diet or soap, detergent, fabric softner, new clothes? If there's no changes in his environment, it could be a viral illness called erythema infectiosum (5th disease), especially if it comes & goes, so you need to take him to his pediatrician for examination & treatment. Good luck.

Allergy: This is a job for your allergist to sort out with a blood test called RAST, and then a directed battery of skin allergens assembled into one big skin field test to place. These should help sort out what common allergens could be causing trouble, and direct therapy in the right direction

Viral infection?: At that age, food allergy or a viral infection would be the most likely cause. If it is from food allergy, it should be gone in less than a day unless he continues to eat the same food. If from viral infection, it may last up to a few weeks and may get worse before it gets better. Try antihistamine up to 3x day. If problem persists, see allergist.

? Viral vs allergy: To food or drug. Allergy testing at this point is not the most needed test, probably RAST blood test, for foods would be warranted, but not a priority , allergy skin prick testing can be deferred with the acute stage of hives, mostly it could viral infection or reaction to a certain food, or due to a drug, start with diphenhydramine, see his doctor now, might need an allergist evaluation later