From just these pictures, it looks like he's standing under with his forelimbs, and - I can't really tell for certain though - he might be base narrow too. The pictures, while displaying a very cute horse, aren't really great confy shots.
I agree, though - he needs more muscling along the topline. Going through the 'timeline' provided by the pictures, it looks as if he just recently lost mass there - the pictures from when you first got him, he looks nicely filled out along his spine. What was he used for before you got him, and how often, do you know?