Your proportions are excellent, well done :)
My main critique is that you need to sharpen a few little details up, and work through your tonal values. So make his nostril, eyes and ears stand out a little more by adding some sharp, dark lines along the focal points. Also darken a little under his face along his neck, to make his head stand out. And last but not least, the forelock needs a little more details ;) Get a sharp, hard pencil and draw a few hairs through it, flicking the pencil to allow the hairs to 'stray' a little giving the impression that there is a light breeze picking up the hairs.