I don't know Parelli, so hope I don't have wierd adice in that context.
You are aksing him to step his shoulder over by putting pressuer there. If you need to, add pressure on his cheek (mental pressure or fingertip tap-tap). If he walks forward I would just keep up the pressure, basically ignore his forward, unless it's into you. If he steps INTO you then tap briskly on his cheek to push his head over and keep the same aksing pressure on his shoulder. He can make a mistake (though not walk over you). You just keep asking until he makes the right choice. Then stop and do nothing and let him soak on it.
I guess I basically look at it as ask for something and if he chooses the wrong thing I don't change a thing, I just keep asking the same old way and eventually he will choose the right repsonse, and he gets the reward of me stopping asking.