When there is an arch such as behind your subject here and you include it, I would use the arch to frame them. In this case I would use a centre composition. The empty arch space draws my eye from the subject.
Nice light and dof, and above is only how I'd do it, not saying you did it wrong
I know nothing about portraiture, but the background seems a little bright to me. It is a little distracting imo. The subject looks good, although a little cold on my non calibrated monitor (might be your style though).
This shot works for guys because of strong lighting and contrast. Like the off center composition. I would like to see more lighting in the eyes. Perhaps a bounce fill from the bottom to add some life to the eye sockets.
This is a nice shot, but I think it would have been better with the subject directly under the arch, and with more space above the head where you have the top of the arch in the frame.
fill flash would have been great here, but you could also try doing some layering in PS and brightening him up a bit, and darkening the background a bit.
on your Flickr page, the shot of him on the park bench was a little better.
Light advances, darks recede...the two white areas seen against the darker arch and him being rendered darkly, and dark-eyed makes this one just not work for me...I can barely keep my eyes directed to him because those two white areas just JUMP out at me, over and over. The lack of head space above him is just not working either. The thing is, I simply must force myself to look at his face...I cannot just naturally look at the image--I have to deliberately, consciously force myself to look at the face, but while doing so, my eye and brain want to, feel almost forced to, pay attention to those two white-painted areas. This is almost like a simple visual perception exercise illustration; my ex-wife was studying Ph.D-level visual perception classes for a time, and we looked at a number of illustrations and example images that showed how high contrast light/dark areas are attended to by the human visual system. In this shot, there are a number of things working very strongly against this shot as a portrait of the young man. The quality of the light, the direction of the light, and the subject matter in the frame are all working against this photo.