Keep a leash and flat collar in your vehicle. Everytime you leave and go out the gate, and the dog is out in the yard, put on the leash and collar and tie him to something near the gate while you are leaving, as well as coming in the gate. Do not leave an unattended dog with a collar on. As he gets older and better trained, teach him to sit and use the "stay back" or a similar command. Do this every time you go in and out of the gate for up to a year of age. You can also have someone be with him on leash, at the gate while you are leaving and test to see when he is learning the stay back command when not tied out. If he ever tries to break the command, he should receive a quick, sharp correction, based on his age and amount of exposure to the command.