The thing about driving horses vs riding is that when you drive a horse, you obviously never want him to turn tight, in the sense that you flex him (nose toward the shoulder, doesn't have to be to your stirrup) and move his hip over to turn.....the driving horse turns wide and generally stays straight on.
You don't want a riding horse to just be straight on and give you wide turns, you want to be able to ask the horse to break at the poll and bend his neck to bring his nose to his shoulder without his pulling against the bit.
It's going to take some time to retrain this horse to think of pressure differently, so don't expect him to be 100% for a few months at least.
What I'd recommend is the following:
Steer the tail, not the nose. Using a basic snooth bar snaffle (no shanks)
1. ask him to go forward
2. pick up the left rein and ask him to bring his nose toward his shoulder (as you drive with both your legs...drive him into the bit)
3. ask his left hip to move over to the right...put more of your left leg on him to tell the hip to move to the right.....you're bending him around your left leg....for a left turn...
4. release only when he's not fighting the bit (be sure to use enough leg to drive him forward into the bit....don't ever kick, only squeeze your calves and if needed roll your heels into his sides) and release when he's bending and has disengaged his left hip (left hind foot crosses in front of the right hind foot)
5. repeat the above with the right rein only.
I'd ride him one rein at a time....direct reining,.....by way of steering the tail for a while, til he's softer to bit pressure
When you can pick up the left rein and he turns left by way of bending his nose to the left and moving his left hip to the right....and vise versa with the right rein, you can then ride him with two hands, and simply close your left fingers on the rein (as you drive him into the bit with your legs) while you open your right fingers and he should give you a nice turn.
I'd go back and forth between using the direct rein this way and two handed steering.
Be sure never to pull, but to take out the slack and drive him more with your legs.
Also, for the leg cue....to turn left, your left leg does tell the left hip to move over to the right, and your right leg tells his right hip to move to the left.