First off, I will say that the image is really too large for viewing on the web. The smaller preview image isn't large enough to see all the details but the larger version is just way too big to comfortably look at. You may wish to resize it to about 900 pixels on the longest side. That's a pretty good size for looking at online.
Other than that, I will just say that the sketch looks pretty good. You may wish to add some darker shading on his face just for added contrast.
With hair t's hard to find the balance between adding enough detail that it looks like hair but not so much that you're drawing every individual strand. Right now I think it's in need of more detail.
I really like the shading under his jaw. And over his chin.