Some anatomy issues. His chin needs to be enlarged. His neck needs to be made much smaller. The nose looks alright. Depending on what style you want, I'd make the eyes a bit smaller (unless of course you're going for a larger animé look). He also doesn't appear to have any shoulders ... I would give him some. The darkest color used for the skin isn't dark enough - since you used it from the hair, throw the darkest shade from the hair into the sprite for more depth (you might as well since you already did it with one color and I don't think you'll be going through any palette swaps). I would rethink the aquamarine used in the goggles. A bit too satured for my taste.
Forgot to mention this - when you're drawing a head/portrait, imagine a human skull. If you look at your original piece, a human skull could not logically fit in that head. It would be crushed/warped. An even better tip is to just draw a skull shape first until it's perfect, then go on to add nose/mouth/eyes/hair and so on. It looks like you skipped that helpful step.