A horse can only be as balanced as their body is physically capable of doing. I mean, if you tried to do what a balarina does, you probably would not be able to do this by virtue of first not having the physical strength and focus. So, your horse needs to develop the physical strength and focus. Your job will be to encourage and push him to work just a bit harder, since he may not have this due to moving in the laziest way possible. It's much easier for a hrose to move while falling on the forhand and leaning on the inside shoulder. They don't have to engage their abdominal muscles as much , or carry their own neck and lift up the shoulder. The rider has to push the horse to work that little bit harder, so that they carry the rider correctly . But with the understanding that it takes time.
There was another thread out there that talked about the use of a martingale and the OP wanting a more collected horse and shoulder dropping and such. Cherie gave several really awesome answers that you should take a look at. Go to this thread and read through: