Hi, this is one of my favorite actors ever!. In general, I think you should try to make a darker shadow that will provide volume to your drawing. Don't forget to press the pencil lightly against the paper in areas such as the eyes, nose and mouth because if you don't, it won't look natural.
I am not an expert in this area, but I have a basic knowledge that I've learned by myself over the years. Hope my comment has helped you :D