aww poor thing! if he's usually active how about some games that involve his hands a lot, one could be where you make up a sequence and he has to copy it, e.g. you clap your hands, he copies, then you clap your hands then tap your head, he copies and keep going like this adding an action each time until one of you forgets, or you could take it in turns adding actions.
you could make musical instruments out of empty bottles and rice, pasta or lentils and start a band.
if allowed to you could get him to decorate his cast with felt tips. hope he's not too bored!