I would say medium bone. He has go a medium in the Tri-Zone Brushing Boots but for those they do more sizes so is more complicated. There is a medium narrow-medium and a medium wide.
if you look on the website i think it gives the measurements in cm of each size in each boot,so if you can find what the medium measurement is in the tri zone in cm, you can compare it to the Equi Chaps measurements.