I was thinking you could create a larger "canvas" in Photoshop, then just clone in more black background around the outside of His Royal Tiny-ness. Should be only about a 5 to maybe 10 minute task, and you'd have a superb formal portrait. I suggest more room just to give that aura of a "royal" or "formal" portrait, which is a style in which there is a bit more top space deliberately positioned above the regal person's head than in a portrait of a normal, commoner. He just needs a bit more "breathing room" around his regal being, you know what I'm saying? It'll really make it seem more like a formal, professional dog portrait.
YES--more room all around!!!! I would crop off a tiny bit of space at the bottom though. And perhaps I would crop just a tiny, tiny bit off of the left hand side too, since he is looking to the right, I think there ought to be, visually, a bit more open space to the right hand side of the picture, to give his line of gaze a bit of room to "look into" as the old saying goes. 480sparky's rework simply gives His Royal Tiny-ness more "space" around his body...it just feels more relaxed and less cramped than the original pic, in which he was close to all of the edges. It's still the same good lighting, still the same handsome dog--but he is simply presented on a bigger canvas!!!
The best way to make cropping decision is, I think, the old-fashioned cropping "L" devices, placed on top of the image and moved around.