I did the same when I was his age went to A&E during the night and all they dud was trickle cold water down my back - anything that cools like this will help
get him in a bath of tepid water, keep him hydrated with lots of sipped drinks of water with some sugar and salt mixed in (if you don't have re-hydration sachets) and get something like calamine lotion or a paste of bicarb and water over the affected places. Slap him around the head and tell him not to be so stupid in future.
The same happened me years ago. I thought the pain of the blisters was bad until the skin peeled and then the terrible itching began. So he has that to look forward to!
elvis I am on my way to bed - you are saying 4 hours in A&E - I waited last year 10 hours and my aunt lay on a trolley for over 24 hours. You are very lucky wherever you live.