Oh, it's a robot! I thought it was a stone golem sort of thing D:

Robots are usually made with more regular geometric shapes than the the loose curves you've got going on here, ya know? Don't be afraid to give him a nice boxy look! It'll give you nice strong planes (flat areas) to work with, which will make shading easier. woo!

You seem really confused about your volumes. All over, but I'm talking particularly about the face. I honestly have no clue what sort of shapes are supposed to make up his face based on your shading. I'm being honest, not mean. Maybe just shade it as a simple sphere for now and decide on his face a bit later?

Anyhow, you need way more contrast to impart a metal look. That's the primary reason I thought he was made of stone. Also, you should avoid pure grays. Give him just a little color. A subtle blue tint makes for a nice steel look.