If you're having trouble defining the subject's form, simplify your work flow. Some things you can do to help yourself when stuck:
- Drastically reduce the number of tones you are using to block in the form. I suggest just dropping to 2 tones, black and white, then moving to 4 tones: black, 35% grey, 65% grey, white. If you can't define the form with just these tones, no additional tones will help you.
- Simplify the 3d form of the subject into more basic primitives. Identify "corners" in the subject - places where the direction of the surface changes significantly. There are many on the skull; the sides of the cheeks and brows, the sides of the nose, the chin, jaw (though these are obscured by the beard, at least identify them to yourself so you know where they are and how the beard connects to them). Break the subject into boxes, spheres, cylinders and cones. From here you can block in the shadow.
That aside, I must second the suggestion to depart from the reference when deciding your lighting and composition. Think about what kind of lighting you want for this piece, low key, high key, above, behind, multiple light sources, what kind of ambient light, etc.
This relates also to the suggestion of "grounding" the subject. Right now it is a floating head... where is it? What is the context? Is it embedded in the ground and we are above it? Is it against a wall? Decide the context, this will also largely decide the lighting.
And of course, have fun.