I can't say that I blame him.
The list of things that you can't do with a full arm cast that happens to extend to just over your knuckles includes:
buttoning your pants
buckling you belt
tucking in your shirt
pulling up your pants*
tying your shoes
returning to a vertical position after falling down on a hill covered in very pokey stickers
washing your un-casted hand
washing your casted hand **
having a sleepover without cracking someone in the head
putting on your socks
carrying any two things at once
*it is noted that while difficult (except for sweat pants) this can be done. The result of attempting without help is a very grouchy 8 year old who requires ice cream. His mother does not object to this requirement and is happy to let him try. Every day if necessary. Practice makes perfect right?
** it is noted that this is probably achievable and certainly easier than washing your un-casted hand, however, the probability of a wet cast makes it forbidden
It is also not advisable to use a port-a-potty in the dark with a full arm cast on. While it can be done items 1-4 will certainly cause a problem and strange things happen. You emerge requiring help, in public, and your mother finds her hand disturbingly damp after assisting. Then she races to the hand washing station.
Some things purell just doesn't handle well enough.