What Causes Red,swollen Rash All On Legs And Back?

My son has a rash that started on his thighs and arms then with a cold shower and being wiped down with alcohol wipes it seemed to go away but instead came back worse all over his bottom ans back and forehead the rash is red with white swelling. Looks like a 2nd degree burn. He hasnt had a fever and doesn't seem to even notice it. What is it?