Tom,
I am having difficulty with this kind of composition myself.
The main reason is the antlers act as a frame and draws our eye to the sky which is not the most interesting part of the image.
The face has details and the green grass on the mouth lends a touch of interest. However, the crop also feels a bit abrupt. You are doing good in handholding the 500mm at 1/125 secs.
I think Sabyasachi has some good points. For me the crop is too tight. I am less bothered by the background but getting him in a different perspective to it might have been better.
A very beautiful image - perhaps tight as Robert mentioned. Personally I liked the dual-toned BG a lot. The only change that I wud do to this image is to provide a little bit of more room towards the bottom of the image.