Very good work -it's extemely expressive (I almost feel personally threatened when looking at it xD ) What you could maybe improve @your shading would be to be more 'strict' with the light source. The most light is on the top of his head but if the light sorce was over->behind him, as the light in his neck suggests, there could not be any light on his snout. The lighter lines at the side of his neck and snout suggest the light is coming from left and right, but that impression is not supportet by the upper part of the head. When you start a painting you need to pic one light source and stick to it without question -and remember that there isn't only light and shadow but also loads and loads of midtones. Practice your value painting and experiment with secondary light as well Another (minor) point is that the highlight-shiny-dots on eyes always where the light is. If you place them at the bottom that would mean that the light comes from there.
Well done! Very smooth and wenn considered shadows that show many Details of the head! Without them, this kind of head would be very flat indeed... so you can say: plasticity came with the shading! And, well, it defenitely did!^^