Pole work, hill work, transitions, and warming up with a loose rein, letting him stretch his neck.
I agree with both Paige and Eq but I would do that all with the horse's poll at least at wither height if not a little lower. Anything with a higher head, if not in absolute elevation, will do the opposite.
Also if you have any hills, hill work is great for building up hindquarters!
Trot poles, circles, spirals, leg yielding, basically anything that so going to get your horses hindquarters moving. For topline main thing would be loose reins because if you shorten them it causes the horse to pull there nose in and "pose" as well as shortening their topline not actually bring there back up and the need to be able to stretch down. Some people think that a curved neck is the horse in a good form but really it's the exact opposite of what we want!