I don't like hard brushes.
By the way, I'm not pushing anything on you regardless of what it seems; I make the mistake often, sorry.
Anyway
It's very difficult to render something with a soft brush straight up like that - or with form, anyways. It is best to block in the colours and values first before you get into detail, and airbrushes are for the purpose of being airbrushes, you don't have to use one to achieve a certain effect. Yes, figure out where the light is coming from, use highlights sparingly and keep a gradient of light, with strength decreasing when at higher distance from the source direction.
Also, his cranium is quite short and flat and the ear is very high, and I'm having trouble distinguishing the planes of the face.
About the left foot: the foot itself seems fine, it's just the position of the calf (which is the only real indicator of the angle of the leg at the moment) makes it seem turned more to the front, which causes the foot to seem a bit twisted. And think of the perspective too! The position of the foot in space aligns it with the right foot, but the legs are not - try standing with your feet aligned and one turned 70 degrees outward, your legs tilted but still in horizontal alignment with your body... it feels unnatural and significantly hard to balance, no?
I could add more, but I should really get back to drawing; good luck!