Your boss doesn't seem defined because the shading doesn't describe his form, it just gets darker toward the middle. From your sketch I can see you're already thinking of his shape being made of different forms, just pick a light source and shade accordingly. Here's a quick, sloppy non-pixel art example of what I mean:
I didn't fix the issue of each "sphere" seeming to be on the same plane, but hopefully this at least gets you to think in the right direction.
Also, your palette has a lot of colors that are too close to tell the difference, you could probably get away with cutting it in half.