Nice looking horse! He does look a bit stiff behind in the hocks so I might look into hock injections, I gave them to my 5 year old and she has 8 inch over steps and moves like a dream! So talk your vet about that.
On you position I would work on getting you hands up higher and really stretching up. When you post you want to have the same amount of effort posting down as you would posting up, don't just fall on his back. You want to be able to stop your post midway down so you can have a independent seat.
For your legs you want to be able to draw a line from your shoulder, hips and heels so you might want to move you legs back more. Also I would really keep you leg on him so you can feel every movement of you horses ribcage. When you put your leg on think of stretching you legs down and back and gripping in between you thigh and calf without pinching your knees or taking them off the saddle.
So to be able to sit up straighter and create more resistants in your body to gain a more independent seat think about your body being an X. Think about you left foot pushing up your right shoulder and your right foot pushing up your left shoulders so that way you stay straight and don't calapse. One way to get the feel of creating resistants with our resisting the movement of the horse is straighten your arm and have some one put pressure on the middle of your arm and try not to move your arm. Most people resist the pressure by pushing back, what you want to do it create your own resistance and point your finger straight as hard as you can and the person can push even harder and you arm should not move.
When you canter you really push with not just your seat but your whole body and your hands go down. The paragraph should help with that though. You horse needs to learn to fallow you. You always hear people say that the rider is going with the horse but the horse is actually going with the rider, be that a good or bad thing.
Hope this helps and if you have any questions go ahead and ask!