I don't think he looks very human. It's something about the shape of the face, the wide mouth and the thin nose. I've found that making faces in front of a mirror gives you a good idea of how the facial features distort, so give that a go if you haven't tried it.
I wasn't quite sure what expression you were going for, so he looks a bit more angry than insane in my edit, but it should give you an idea of what I mean. The AA on the hair looks a bit fuzzy and could use some work, too.
Oh, and a note about dithering; when you're using 3 colors like that, put the brightest color next to the darkest. It'll make it blend better and help you avoid banding.