My grandmother went through the same thing when she broke a couple of ribs and her arm between the shoulder and elbow. Because of her age, they had to put her in a stabilized sling to keep her from moving any part of her arm.
She chaffed badly!!!!
The reason? Because she was sweating where skin touched skin (like under her breasts and armpits or anywhere her cast met skin).
The treatment for any kind of chafing is to buffer the skin so that skin-on-skin contact doesn't happen. Powders DO NOT help!! As a matter of fact, powders contain an ingredient that irritate the skin more and create heat which really doesn't help if there's a rash going on.
The solution was to clean the area very well, dry it very well, then put an anti-chafing cream (like Monistat or Simply Delicate from AVON) on the areas where skin-on-skin contact happens. It goes on silky and dries within a minute and there's no gunky mess at all. I continue to use this under my breasts because it really works to keep rashes and irritations away. (I also use it as a make-up primer, believe it or not)
Also, you might want to use a milder shampoo (like Johnson's Baby) and a sensitive skin soap with no deodorants or perfumes to keep the itching down. Try a garbage bag over the splinted area to keep it dry when he's taking a shower. Dad might want to help out washing and bathing his back and legs but your son might want to take care of his own 'boy' parts and such. Try putting him on the kitchen counter to wash his hair as lying back on something would relieve tension in his neck, arm and shoulder from his splint/cast. Basically, give him a day spa treatment when cleaning him and have him be as relaxed and comfortable as possible. He's going to be in a lot of pain for a while, poor thing!