Does this really not look like Piett?
Maybe it's the skin tone and lips. Maybe he should be more pale and strung-out looking. A repaint would really bring him out I think.
I think an alternate position for the eyes might change things too, maybe a dark wash to help define the eye sockets even when they're brightly lit.
The thing I find about likeness and lighting with Piett is that I always think of his face as sick and unhealthy looking with his eye bags and all, and I find under some lighting, those features don't stand out as much and the portrait looks healthier than the actor.