It looks like they built up a lot of the facial features with either fondant or modeling chocolate. It looks like they trimmed a cake to size, put down the fondant/mc to build up the brows, nose, mouth etc, and then covered it all in green fondant.
Ditto Bellatheball. I would bake a square, carve not only into the round shape of the face but to allow for depressions in the face, then crumbcoat, then build up with fondant around the "cheeks" or any other high areas (lips, eyes, etc). Then lightly crumbcoat again and cover with a large, green and slightly thicker sheet of fondant (thicker than usual)--the extra thickness will allow you to be able to press down into depressions without the fondant tearing.
It also looks like there's a bit of shading happening on and around his nose, cheekbones, etc. (wherever it looks darker green/brownish) which is giving him even more dimension. You could A) Dry brush with a foodcoloring/extract solution or B) Airbrush (even if it is the Wilton spray cans-- basically the same thing & probably easier).
HTH and don't forget to post a pic!
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