If the horse doesn't respond to leg cues then either he doesn't know them or he has decided that the human doesn't know them.
Here's the progression:
1. Teach yielding to pressure.
2. Establish the buttons--3 buttons on the side of the horse--front buttion controls the front leg--middle button controls the ribcage--back button controls the hind. Clinton has a nice method for this.
3. Learn how to cue the horse correctly--using seat pressure and leg pressure is the key to good riding and takes some time--and money--to learn.
Put the two together and bingo!