I actually really like what you have so far.
But, you are right that the shading isn't quite realistic (though it is still visually pleasing, at least to me.)
Here's a good reference image for you.You're not that far off, but I think your colors on the ridge need to diffuse and meld a little more, and you should perhaps try fewer bands. Maybe have four bands total of dark/light/dark/light.
Also, I don't think the very back should be in the dark sort of shadow that it is. Metal is highly reflective, and with light casting onto the top of the helmet and ridge, I imagine that light would reflect back onto the back of his his helmet, even if it's not in direct light.