Don't use the reins, use your leg. Hold both reins steady (even pressure). Wich way does he shift? If he shifts left, you need alot of left leg. If he doesn't respond, use a little bit of LEFT rein and left leg. If you pull your right rein, he will just bulge his body more left. If he is a right shifter, reverse this
If he doesn't have a specific way, just hold both legs and both reins evenly and LOOK UP. It is amazing how much straighter they move when you look up.
A helpfull excersize is to put a pole (on the left if he shifts left, or visa versa) . Place it so it is about two feet from the middle of the jump, perpendicular. This will help you understand where he needs to be