The Radian will work well shape-wise, but keep in mind that no seat will fit him sitting flush with the shell and that's OK. Pad the gaps and the edge of the cast at his back so he's supported enough not to get pressure sores or rub burns.
The failing of the Radian for casted kids is that the harness may not be long enough. It takes up a ton of extra harness across the thighs to get around the cast. If the child+cast weighs little enough to use the safe stop, you have a much better chance of it working.
If they need to buy a seat, I have the most success with the Triumph and casts. The Foonf has the longest harness of any seat according to a thread where I requested people measure, followed by the Complete Air and the Nextfit.