For materials, what about wire?
Thin-gauge white solid wire (the solid part is important, because stranded wire is too flexible to hold a shape) would have the right sort of look and probably suit the ability of a kid his age. Just let him go at it with a pair of needle nose pliers and he can cut and bend the "bones" to suit.
It'd probably be a lot less messy than other materials, and it doesn't dry out like clay while you're working with it.
It wouldn't work for broad bones like the pelvis or skull, but it's strong, stiff, easy to use, and easy to straighten if it's bent too far.
If you don't have an electronics supply house near you, then the Source will probably sell it... look for something like this:
ONE-CONDUCTOR SOLID HOOK-UP WIRE | Hook Up Wire | TheSource.ca It might also be sold as "breadboard wire".
Or, if you know any electricians, they might be able to give you a whole whack of wire scraps (though that might take time that he doesn't have now
).
Anyhow, it's not a traditional material, but it might serve your purposes.