Well, firstly it's important to understand that Snake has artistic knowledge that goes beyond pixel art. He understands the structure of what he's drawing, and the way light works with it, on internalized, instinctual level. Trying to emulate him without possessing this knowledge is like trying to swim without arms.
The best advice I could give you it too pick up a pencil and sketch book and go outside and look really hard at things. If you really want to stick with pixel art, and it's not the best place to learn, but maybe look at the way he has split the rocks into individual planes of colour in order to give them depth, and try to emulate that on your own.