yeah, that's a nice shading... but ther's a redundancy on the black colour. Infact there are two different black graduations in the image: you should fix it...
By the way... the facial expression is some kind irregular and the cyrcle shape in the back lacks of roundness...
Have you any reference for this image?
this was a fast and simple, not given much attention, certainly I will make an update;)
and there was no reference image, I just remembered the character and his sarcastic smile = D