DS was in a plaster cast that was changed 4 times when he was a year-old - not quite serial casting, though.
Not needing to cut the cast off with the circular saw is a huge benefit - breathe a sigh of relief, there! It terrifies most children!
Will they teach him to use the frame and let you have it to practice with before the first cast goes on? That can be v helpful. And if he will need to keep the cast dry ask in the fracture clinic about the waterproof plaster coverings that can be used in the bath. Does he enjoy a bath? As much as can be kept as normal as possible will help, i suspect.
Have you seen the STEPS website and forum? There may be parents of children with the same ocndition there.
DS was funny with his leg in plaster - he was absolutley fine when it went on, just adapted, and was clulnking about, crawling, just as he always had.
But when it came off he was very jittery. He didn't want to put his leg down, and seemed more perterbed by it coming off than going on - so be prepared for that in due course.
Good luck - i hope it goes well.
oh - and don't let him 'post' toast down the cast......that was a nightmare!