working on exposure /lighting
The subject exposure looks pretty good, close to Rembrandt. The fill might be a bit strong for my taste but not by much. You might let the lighting fall off a bit more from the face downward. The biggest problem is that the hair looks dull & dead. A hair light would fix that.
You said not the pose but lighting and posing go together. The pose has the subject broad lit, which makes his face look wider (fatter) than it really is. Rotating the face to the subject's left until the ear just disappears and adjusting the lighting to again give you Rembrandt lighting would give you short lighting and be more flattering.
Don't let the dark upper corners worry you. When doing seamless white all you need to do is get a pure white halo around the subject. It is easy to fix the rest of the "white" area in post processing. Of course if you can get the lighting right so that all the background is around +0.8 ± 0.2 stops then you won't have to fix it in post processing.
Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.