Hills will help a lot my horses have so much muscle tone from just going up and down the hills in the pasture and there just brood mares but they look to good for brood mares it's crazy a
Lunging him once a week. Lunging is a great exercise, do it on varied terrain. Also use some accessories like chambon, correctly, is a good option.
The most important with a young horse, is making him going forward! Before asking him to relax, to be on the bit. If the horse doesn’t take the habit to push on his legs, to have propulsion, you’ll have some problems later. When the horse has propulsion, the head isn’t a problem anymore ☺
The most important is the propulsion. But it depends on how old he is and how his dressage is. But first of all you can do many transitions and curves. When you ask curves to your horse, he’s going to be balanced by himself so ask him propulsion when curves, and so it’ll work his back ☺