First of all, you're horse is great!
Second, the head is way to far back, if he loses his balance an falls he can break his neck like this! So try to keep his head more in front of him except under his body.
Third thing, as you can see his stretched leg isn't stretched but curved :P wich needs to be straight. He hasn't got enough space to move his weight to the back. A good way to fix this is to learn you horse to stretch (I don't know if this is the correct word in englisch, in dutch we call it spagaat). It's like in this picture: http://18.104.22.168/1360900001-136...43_5_a_Vd.jpeg
The horse on the picture is stretched veryyy much. Your horse doesn't have to go so far to let him bow, but just a bit to create some space for him.
Here a picture of a correct bow: http://i51.tinypic.com/2qd3889.jpg
Fourth, subescribed :) Really beautiful horse!