Hello, the idea for the painting is very interesting. I like the composition - face is placed in the thirds of the painting - maybe I'd just crop a 60-70 pixels from the right side for better balance. The starting colour scheme is quite nice for me - I'd just add some red maybe. I see you're looking for detailed comments about your artwork so for me there are two major things that could be improved on your artwork:
I've quickly prepared the reference so that you can see what I'm talking about. Using reference for light and pose might be great idea as well in every occasion.
1. From the point of view that you're using there shouldn't be visible a bottom part of the right arm (it would be just impossible to bend palm like that). Also the size of the hand should be about 50-70% greater from what's currently on your drawing and when you define the folds on the arm it should be visually thicker. If you decided for the camera placed lower than the eyes of the figure it would look scarier than when seen from top.
2. I can see overall flatness of you lighting scheme. On my reference I've used 3 point light scheme: http://3drender.com/light/3point.html
Key light is placed near his face, near eyes, above the noise, fill light is soft and it's placed above his right arm, rim light is the distant light, on his left side, slightly behind the figure, added an fx light to make interesting point inside his hand. You can also notice that all these three lights have different colours - that really captures an eye of the viewer.
I hope you've enjoyed the tips posted.