First of all you need to know if he doesn't have a tramatism due to strong hands or strong bite.
To make him draw reins you can try a heavier bite (not a stronger one but heavier, one that is full and not hollow).
Concerning your position, you need to put your hands forward when he overbents and keep your reins tight (not fluctuating, keep a regular contact). If a horse doesn't draw reins, it means he grows hollow his back and doesn't push with his posteriors, you need to keep a lot of legs, make small actions on the reins to make him release tension.
For the exercices you can do, you can try shoulder-in to engage the interior posterior and ask him full neck extension, this way he will come and look for the bite himself and you won't have to ask him because he'll find it comfortable.
I hope i could help you :)