In addition to the issues pointed out by Sam, IMHO, the first thing that screams,
"Caught you: Photoshopped!" is the sharp line that separates the top of the subject's head from the background:
This tells me that this image is either (1) a full blown composite, or, (2) at minimum, the person was treated differently (in PS) from the buildings, but the selection of the person wasn't done well.
The second thing that caught my eye was that even though you attempted to convert everything to grayscale, you left a bit of color in the image. I doubt this is intentional. The color that remains isn't very obvious, but to me, it screams,
"Photoshopped!!!", whereas if an image was either left in full color or converted completely to B&W, these options are more common (ie, than just a trace of color remaining), and are accepted by almost all viewers. The remaining color can easily be seen by cranking up the vibrance and saturation controls. In the image appended below, I also attempted to minimize the black line / selection error discussed above:
To be honest, I prefer a color version to a B&W version of this image because (a) color better separates the person from the background, and (b) color (if done properly) can distract the viewer's attention away from errors in the image.
With respect to the point Sam raised about the apparent difference in perspective between the person and the building, thereby suggesting this is a composite image, personally, I can't be sure about this because, in principle, the person could have been leaning inwards towards the camera, and this would have made his verticals converge less than those of the buildings.
HTH,
Tom M