I always recommend cast for young kids with fractures, even non-displaced ones. Kids are absolute animals usually and have little to no self awareness to not make things worse. The consequences of that thing shifting could be life alternating as well. Could he do fine in a brace? Probably. It's not worth it though in my opinion. It's a few weeks of an annoying cast for a much higher likelihood of a well functioning wrist for the rest of his life.