Brief Answer:
Looks and sounds to me like an allergic reaction, need to isolate cause

Detailed Answer:
Hello and welcome.

Thank you for writing to us.

I have gone through your query with diligence and would like you to know that I am here to help. From the given images I strongly suspect the sudden onset of this rash which could be related to an allergic reaction, please find some time to respond to the following questions for my better understanding:

+Did he consume anything a while prior to the appearance of the rash and/or itching
+Did he come in contact with substances like pollen, new sheets, new cosmetic, new detergent used on the clothes that he wore just a few minutes prior to the appearance of the symptoms, new deodorant, new perfume, etc.

+Basically anything new he used, consumed, or that he came in contact with just a few minutes prior to the symptoms that appeared

For now, please do the following:

+Try to isolate and find the cause for the allergic reaction and avoid it completely
+Try to give your son any anti-histamine you can get your hands on either at home or from a medical store (preferably pheniramine, chlorpheniramine, etc.)
+Give him plenty of water
+Coat his skin with a lotion like calamine lotion that will help keep him calm and soothe him to prevent scratching and/or further irritation
+Give him a mild pain management medication as well

I look forward to hearing from you in order to help you further.

Best wishes.

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