The renders colors just don't match up with the background. Looks kind of "thrown on" at the last minute. Not too bad though.
He's not a part of the background at all, and that makes it look flat. A regular b/w gradient might look better.
Even with the bw gradient, it still looks like you threw a render over a random background. There's no flow, and it's just not blended. Plus the render is so choppy, that it makes it look bad. Honestly, I'd scrap this one and start over anew. No flaming intended, I just think you can and will do better.
