What Could Rashes On Hands And Arms Suggest?

Hi, may I answer your health queries right now ? Please type your query here...My son is 15 years old. He has a rash that started on his hands(palm sides only) that then traveled up the insides of his arms in hours. He has a doctor appointment, but not for a few hous. What could this be, I've never seen anything like it. It looks like sunburn/diaper rash, no bumps.