Much of it will be management. Since he enjoys chewing on them, it is a self-rewarding behavior. Keep an eye on him constantly in any area he has access to rocks. Try to keep him engaged with you with a game or training when he's in the backyard or any area he may find a rock. If he does get one, remove it and redirect him to an appropriate chew or toy. Work on leave it with him and start working on leaving rocks alone. I had to do this with mine and Kleenex. It takes a while, but if you are vigilant, remove anything he gets a hold of, and work it as a leave it, you will be successful. Mine won't even look at Kleenex now. It just takes some work.