A looooooooooong time ago, about my 3-6 casts ,a Doc. told me NEVER put something down my cast. And a min. latter said that I could use a mix of cornstarch and baking soda. The cornstarch sooths the skin, and the baking soda helps with smells, like in a fridge. It has worked for me and my kids, they have fallen off the monkey bars at school( or even out of their chair!) a few times. Heck I twisted my ankle and got a concushion just walking out the front door! ( purse strap caught the door handle) .
Best of luck.
Do NOT let him scrub his arm when he gets the cast off. It feels good, but you can really, really damage the skin and get a lot of small ugly scars. Did this, hurt bad, not fun, felt good for the first few min. of scrubbing.