Great model, but your polypaint its letting it down... Try texturing him the conventional way using Photoshop and textures instead of trying to make it look good on Polypaint, i think your main problem while doing it with polypaint its that you are not using any textures. you are simply painting with base colours.
i'd start by getting a base down in zbrush using the polypaint tools. just to get a rough feel for where you want colours. export that data, overlay your AO, and start painting on top =]