So far the only real issue I see is that the lines on the forehead seem more like scribbly lines than part of his face. I think more definition
using value rather than line would help in this situation. I've seen a style similar to this before, so I'm not sure if this is your style or what have you but the shading around the jaw seems a little too "layered" compared to the rest. perhaps less colors and more definition using shape in that area would help? Good work I really like the shading around the eyes!