Hello,
Due to the suddenness of symptoms, it can be a viral rash or an allergic reaction. I suggest you to ask him to consult a doctor Immediately. He needs antihistaminics like Benadryl and topial and oral steroids for this which are available under prescription.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
There is no way to help you with this question...not safely... and with him in Thailand.