The cause of the hives can be drugs, foods, stress, and is unknown in many cases.
The fact that the rash appears anywhere on body suggests this diagnosis.
Hives are an allergic reaction and those with asthma and allergies are more prone.
If the cause of hives can be identified, you should avoid that trigger,but since you don't know the cause then try to remember or make a diary of what your child eat at that day or what medicine he takes or what is new in that day so he becomes allergic . Some drugs and foods take days to leave the body.
For the mean time you can give him antihistamine to relieve his symptoms, but it is so important to figure the trigger for the other times .
Contact dermatitis is another possible explanation for the rash. This rash occurs when certain substances come into contact with your skin. Contact dermatitis causes a localized rash on the skin which may be red, sore, itchy, and have bumps or blisters. There are two kinds of contact dermatitis: irritant or allergic.
Irritant contact dermatitis is more often painful than itchy, and is caused by a substance damaging the part of your skin where it comes into contact. Contact with acids, soaps, detergents, fabric softeners, or other chemicals may cause irritant contact dermatitis. The reaction often occurs on the hands and is frequently work-related. So try to remember what new perfume ,soap any thing newly used or everytime your child develop hives is used .
Allergic contact dermatitis is caused by exposure to a substance or material to which you have become extra sensitive or allergic. Common causes include fabrics, clothing, soaps, and nail polish. Rubber gloves are one of the most frequent causes of occupational allergic contact dermatitis, especially in health care workers.
So give him antihistamine if mild if you think the condition is severe (swelling of the face beside rash then check with the doctor ,some allergic actions might need more advanced treatment to dominate the condition .
IF the condition happen every time and you still don't know the trigger visit the doctor to order some allergy tests to know the triggers .