Suggest Remedy For Burning Rash On Arms And Biceps

Hi. My family & I were at a lake swimming today & our 7 yr old son started getting a rash under his arms & biceps. After awhile it looked like a chemical burn. The lake manager said they have been treating the algae in the lake with copper sulphate. Could that be the issue? It is burning him & putting Benadryl spray on it just makes him scream in agony. Right now the only relief he is getting is from wet paper towels on his arms, he said the coldness helps. Any information you could provide would be appreciated.