You can put ice bags on it to help cool it down.
When I had my waterproof cast on my entire leg I would get in the shower and soak it down then pour a little shampoo in and gently move the cast up and down. Suds would come out everywhere!
I would rinse and rinse it until no more suds came out. Then I would use a blow dryer on low heat high fan and dry it or sit outside in the fresh air and let it dry. The waterproof casts/fiberglass casts are wonderful and of course not meant to soak in water but they can handle being wet at least once per week if you take good care of it afterwards and get it dry.
When it came off I had zero dead skin and my skin hardly ever itched.
I'd say if he's itching so bad you had to give him Benadryl that he may be allergic to the substance touching his skin. That's worth checking out.