You have no idea how difficult it is to provide you with some constructive feedback on this image. As far as I am concerned, there is little to improve upon and I am at a loss to figure out how it didn't score over 6. My only conclusion is that you got dinged because it was just a straight black and white.
The composition is excellent, the tilted angle works exceptionally well here. You have cropped it beautifully, the depth of field is lovely and the catch light in his eyes is gorgeous.
As I said, I don't really have anything to give you in terms of how to improve, as I think this is an excellent portrait.
Well done, I'd be delighted to have such an image in my portfolio.