Thank you for the feedback, those are all good points.

Pretty good. A few things I would look at: 
1) The sudden change between the grey brown hair and grey beard.
2) The hairless (asymmetric) space below his lower lip
3) At a glance, you're working with a very restricted palette, which explains why his hair has no highlights. But in reality, you have 27 colours and many of them feature only in less than 15 pixels.