He's beautiful! I think the colors and detail makes it one of my favorites of yours. The lighting on his face and shoulders is perfect, and I love the expression. The background also compliments his colors very nicely, and I like the subtle watercolor look to it. My only suggestion would be to define the shadows and highlights of the mane a little more. It looks a bit overworked and grayish, so emphasizing a few dark areas and picking out some highlights might help. But it's looking awesome so far! I can't wait to see if finished.
It's magnificent and you are an amazing artist. I'm not, so I say this with all humility. To my eye, the patch of shadow on the his chest seems a bit dark on the lower left hand edge. It's probably right in the photo or whatever, but it seems to detract from his underlying anatomy. It seems too harsh an edge which makes his chest look a bit of a funny shape. To my eye it detracts a bit from his real lovely curves there (it is such a lovely bit of a horse). I hope you don't mind me saying this. Maybe that bit, level from where the rein disappears back to the full sun, could just be lightened a little bit?
He's looking very lovely so far! As far as the head goes, maybe you could give him a bit of a roman nose? His head is probably a bit too refined at the moment, so even making his nose a little thicker may help too.