jimjams, if he has a fracture, it won't necessarily NEED a cast- a lot of children's fractures don't. There tends to be a knee-jerk reaction in casualty; it's broke, it needs a cast, but often this is not the case.
If you do go to A+E, and if you manage to get him Xrayed, then if there is any suggestion of a cast being needed, explain about his background (they'll probably have an idea by that point!)and ask if there's any way a cast can be avoided. You will probably need to see an orthopaedic registrar before you get someone confident enough to make the decision!
A backslab is made of Plaster of Paris, and is half plaster, half bandage, it cannot be walked on (well it can, but you shouldn't), but is relatively easy to remove (nurses usually use scissors to cut the bandage).
A fibreglass cast is stronger, designed to be walked on, but usually needs a saw to remove.
A few places will have "Softcast" which can be cut with scissors, but needs to be reinforced with fibreglass (could be a good option).
Depending on the break, a tubigrip or splint may be all that's required.
My dd broke a bone in her foot last year, and although she had a cast, she would have got better without one!