Well just my own ideas, but... I think I'd give him more space at the sides and quite a bit more at the top - say 2 or 3 times as much space at top as at bottom. In general I think the eye likes more space above the subject than below. And it this case it would emphasize the subject's short stature and so add something to the composition.
One mistake I've made in the past is not allowing for some loss at the margins when matted and framed.
Who is this guy anyway?
Here's one of mine, with space at the top making him look small and, well, cute. I think it works, but that's just me. Maybe it doesn't work, with your subject.