Wow, very nice dramatic portrait! The only thing I would suggest is to bring out his face more with stronger, meatier color, perhaps using a wider brush. You have that wonderful loose stroke that can be quite effective here. Also, dab a tiny, brighter highlight in the glint of his eye nearest the light source. (Don't make the highlights the same in both eyes.) Right now, my eye goes to his shirt, and I would prefer that to be the second reading.
