Very nice portrait. I can see nothing wrong with it. Great job.
The first thing that jumps out at me is that the head seems a bit stretched. Here's what I mean:
I get the impression that you started on one end of the face, and worked your way to the other side. What might help is to start by drawing a circle use that as your frame of refernce to keep the head in proportion.
Hey dude, you need to give me a time frame in which you have done this drawing. My critique is heavily dependent on how much time it took you to draw that face!
I think it may have been 20 actually. Any recommendations on how I can improve?