To me, it just seems as if you're doing thick line art. You may think just because it's white and black and a little rough, it looks like Bill's work, but from what I can see from the example you posted, it doesn't. What I see is him conveying lines, shadow, texture, etc. through the use of just 2 colors. Try looking at the helmets between the 2 pictures for a good comparison. It's a good start, though.