All of a QBs power comes from his legs and core. The best exercise to work both of those things, is squats.
A QB pushes off of his back leg to generate force. That force is created in the squat rack. If you can build up a strong lower half you will be able to improve your velocity on the ball. Also your distance on the ball will increase.
Throwing is an all out effort exercise. You are letting that ball rip every time it leaves your hand. So thats how you should be training squats as well. Lift heavy. Build up the power in your legs.