I think the head is well constructed, though the white lines don't feel like they are adding anything to the face. Also, the treatment of shadow and hair feels a little sloppy. The eyelashes' value, though in shadow, is somehow brighter than the ones in the eyebrow, which makes it look strange. Still, the face has a good expression, and the drawing is very solid.
Since you said this is a skin tone exercise, I guess you wouldn't worry too much about the rest of the figure. Still there are a few things that worth pointing out, I think. The folds on his clothes really needs more work. To be honest, I think every folds you have on his jacket are folding the wrong way. The best way to do this is use a reference and see how folds in those area behave. I assume that the shows and the pants and his shirt is all one piece. If it is made out of a big piece of plastic material, then it looks nice, but I would still watch out for those folds on his pants. I'm not sure why they all form these razor sharp edges at these folds.