At age 10 a 26″ wheeled hardtail with small frame should do nicely, depending on how tall he is.
We started our boys on xs rockhopper frames, really nice ride. Used 14″ on one scandal too, but too long in the top tube for kids.
I had no problem with 170 cranks for my 10 year old. Probably not perfect, but then my boys have quite long legs. 13 year old now wears 32″ length trousers, they soon grow! My oldest is now on his 3rd 26″ wheeled hardtail frame.
My oldest actually outgrew one frame before I built it up! Bought him a knolly xs frame for the alps just after last years trip (when he still used his norco b line 24) , but by the time I got around to building it up he had outgrown it. His little brother loves if though….
Simone