Thanks for those helpful tips, I've made some improvements on my creation based on your suggestion.

The sleeve on the left has some jagginess to it, and your outline varies in thinkness from place to place.

His chest seems rather undefined. Everything else has sensible shading, but the chest seems to be pillow-shaded, with shading on all sides and no sense of form. You could probably get away with just making the whole chest in shadow, or having just a little bit of the midtone on the left.