Not really as extensive as Parkerbaby's edit, but here you go.
-At this resolution you don't really have room for much detail (e.g. creases in forehead), focus more on readable planes, like his prominent forehead
-Try to sculpt the forms more, right now you're mostly outlining them, for instance the nose. It will help it look less cartoony.
-Right now there's ~27 colors, most of which are very similar. Try reducing the palette to 10 or 12 (that's something I didn't have time to address in my edit)