My eldest had exactly the same thing when he was 3, serial casting and night splints for toe walking. The things we found most useful were:
Adult size slipper socks to go over the casts at home we have a resin floor in the kitchen and tiles in the bathroom that were dangerously slippy for him to walk on in his casts - grippy soles meant he was safe.
We just did a 'sink wash' until he needed a hair wash and then we taped bin bags over the casts with sellotape (on the bin bags to make a gathered 'cuff' not on the skin) to prevent splashes and then DH and I dunked him in the bath so that his knees were bent over the edge and he was flat on his back in the water iyswim?
He was given 'cast shoes' by the hospital (velcro sandal type things) which were great for round school but gave no weather protection and your number one concern is keeping the casts dry so we bin bagged over the casts with cast shoes over the top for travel to school. Bin bags removed as soon as he arrived though or otherwise he'd have stunk!
You may want to get a cheap pair of black jogging bottoms from the supermarket and cut the elastic cuff at the bottom in case he can't get his uniform trousers on. Or if he's more confident you may be able to get his summer shorts on??
He stayed inside at playtime if it was raining or there were puddles on the floor.
No PE (at schools insistence - not ours)
If you walk far regularly e.g. We did quite a big school run daily and you have smaller DC you could consider a buggy board - as he's older he might not go for that, how about a tow on his scooter if he has one and you can manage it?
We also got him using blankets rather than his usual duvet at night as he got too hot on his feet.
School will have dealt with kids like this before and should have an idea of what's sensible but it may be a good idea to go in and speak to them anyway. On a separate occasion DS had a broken wrist (oh yes all but one limb so far has been in plaster before he's 6) and he was allowed to play in the sand tray in reception class as we had only specified not to play in the water tray D'Oh!
We always had our cast changes very first thing in the morning - they said they liked to get kids in and out before there were lots of injured people in the waiting room. This meant DS met one of his now best friends, a girl who was a few weeks ahead of us treatment wise and we got to see her having her casts cut off so DS wouldn't be worried about it. As it happens DS giggled his head off as he said it tickled every time he had a cast off (some others can be frightened of the sound of the 'saw')
Oh and the last thing - you will be massively worried about this and how it will affect him. I can almost guarantee that after the initial its not fair no playtime in puddles he will not be arsed in the slightest. My DS loved the minor celebrity status in his primary school from having two casts!