The big nitpick with this one is the upper part of the nose. It curves in too much, which makes both his forehead and nose seem like they're protruding way too much. It's a pity, because it's a good looking nose, it's just not quite the nose that fits with the rest of the picture.
The other thing to work on is where the lighting falls on the face. Shading faces is tricky, but I'm noticing that getting the shading right is a major step to good pictures. Draw very light pencil boxes on the face in the areas that should be shaded, and then colour them in. Follow the lines of bones and natural curves. It adds a lot.
My recommend with this is draw the head in it's entirety first and then add the shading. However, I'm generally behind on my pencil sketching and can't offer much advice as to actual shading techniques. Try looking at actual art books.
Also remember that running your thumb over pencil sketches smudges the lines and makes much softer shadows. I also don't see any hint of a skeleton there - you'll notice that people draw a circle with a cross through it when they're drawing heads and, honest to goodness, they do it for a reason.