By "on the vertical" do you mean on the bit?
If so, the first thing to focus on is the impulsion and the engagement of the hindlegs. When both are mastered, you can do simple exercises (circles, figure of 8, serpentines,...) with your reins tight and a soft and regular contact with the horse's mouth.
If he resist a bit, use your external rein and your inside leg.
Be patient and reward him every time he behaves good.