Yep, what Rocifier said. A lot of his face sits really flat, this is partly in the contour of the face (it's silhouette), and also how you have shaded the sprite. You need to think of your sprite as a 3d object, where you place the highlights and shadows will effect how it reads as a 3d form.
I did a really quick edit, removed most of the dithering as I don't think it's needed here. I pushed both the min/max brightness levels a bit, making the shadow tints darker and the highlights a little brighter.
I also increased the saturation of the skin tone a little bit more in the highlight & mid tones, as these areas are (presumably) getting light from the sun, they would be warmer, while the shadows slightly would lose their saturation.
