I suggest you immediately take him to a dermatologist
or if they are not available, take him to emergency department in nearby hospital.He is probably having acute urticaria
.well there are lot of other causes which can cause a rash
over the body.It is very importantthat he be seen by a doctor who would be able to take a complete history and do the necessary investigations so as to treat him appropriately.
Meanwhile if you have antihistaminic syrup at home you can give him the recommended dose and put cold compress or calamine
lotion over the rash.
Hope it helps