My grandson is 17, he went swimming Thursday and got sunburnt, it seemed to be getting better, it s only on his arms and back.when he got out of the shower around 10:30 Saturday night, he went crazy, saying it felt like pins and needles and itching and burning, he was screaming and crying, saying it was the worst pain he s ever had, he said it s so bad, that he wants to rip he s skin off, how long will this last, he took a benadryl and I put a wet towel on him,please help!