Work on the teeth, they look a little like kiddy like a caveman in one of those weird dinosaur cartoons. Make them darker, and yellower.
Make the eyes look brighter as if they are slightly glowing, make them have a yellow tune to them.
Make the top of his head wider. It looks very irregular when the top of your head is much smaller than that of the bottom of your head. Also, you might want to get rid of that yellow color on his head. Looks rather strange.
Other than that, its a good start. I'd suggest an update