Pole work, hill work, transitions, and warming up with a loose rein, letting him stretch his neck.
Hill work helps along with tons of transitions!
Long and low is the way to go!!
walking up and down hills can give you a nice base to work with
Doing transitions and engaging the end, also trot poles:)
And riding in low head position and maybe some lunging ;))
Passage from gallop to walk(and from walk to gallop) and form stand to trot (and trot to stand)