Well, I have to say, this peice is beautiful! Your shading and colouring skills are beautiful! The shine on the mane and tail are very realistic, infact, i wouldnt change anything about them! However, A few things I would like to suggest;

On a real horse, when looking at conformation, you should imagine an invisible line where the girth should go - on this pic about a cm back from the base of the forleg - there should be the same amount of horse infront as behind. As you can see, Will's rear is longer than his front.

For the next point, His fetlock is a tiny bit long on his front left - the shape and actual joining to the hoof and leg are perfect, just long

Okay, so next, lets look at the legs. If you image a horses leg only has limited movement; you will see that his fore right leg is kind of awkward. A horse could have his hoof there, but theleg is slightly too far forward at the base. If you image he has his chest facing away from us slightly, his right leg will look further back... Does that make sense?? lol, probably not! Anyway, as I was saying, the back legs are great - maybe his hind quarters are a smidgen big, but nothing too major. okay next, if you try to think, that both his front legs will be the same length, and the same with his back legs. His front right is actually longer than the other, but that can be solved by moving the left joint lower!

Hmm... my last point would be to make the head bigger and the neck longer... Thats about it though!

Sorry this was kinda long, its only tiny things I just wanted to point out I love this peice, its absolutely gorgoues!