Instead of trying to create an ability that allows the cavalier to force his focused target to deal with him, why not give the class abilities that let him make himself much harder to avoid. E.g., movement through threatened squares, batting feeble sword swings from mooks aside as he closes on his chosen foe with single minded determination or being able to charge through occupied squares (bonuses to overrun or being able to overrun multiple foes), heedlessly trampling those foolish enough not to dive for safety as he closes in upon his hated enemy or a big bonus .
Instead of forcing his opponent to stick with him, let him just be much harder to get away from. Maybe his dedication is some complete that at higher levels he just ignores some kinds of magical barriers (or gets a save to ride through them or what not) and movement impairing effects.
Also, if the goal is to have him challenge a target and then engage it, it could be facilitated by mechanics that reward closing with the target (while at the same time enhancing his ability to ignore others and pursue his chosen foe) (e.g., a scaling bonus to AC if he ends his turn closer than he started to his chosen foe)
I'll leave it to better rules-mechanical minds than mine to come up with what would be balanced, but hopefully you get the general idea.
