Note that in the image above, the dark outline and true CrossHatch shading have been omitted to clearly illustrate the Perspective Camera projection of the grid texture onto the surface.
And this is done exactly as Leonardo does, you can look at the way the pen moves next to the contour to make this very fine grid of parallel shading and it's absolutely what Leonardo does.
Here we have a 10-year-old boy with a strong jawline and overall good looking face...who went on to develop flat cheeks, a receded chin, a weak jawline and a slight hook in the nose by the age of 17.