Most of all, let him have fun. Let him know that the objective is to shoot game, but how much you shoot is not the true measure of a successful outing. Remember he's only nine years old so he will tire quicker, and get bored sooner than an adult. He will also have a lot harder time sitting still than an adult. So don't be hard on him if he's playing' with his shoelace instead of sittin' still and spooks a squirrel. Whether or not you see any live targets to shoot at, give him the opportunity to safely shoot his gun at a still target without pressure, so he can be successful. The tips about food and snacks are accurate. I've found over the years that spending time alone in the woods with your son gives you the opportunity to talk about things that are uncomfortable to talk about at home. After sittin' in the woods for a day and talking about everything else in the world, it's easy to bring up things like sex, alcohol, drugs and smoking that would be awkward at home. Discussing the reasons and methods other animals use to propagate their species makes it easier to discuss and identify our own human methods without near as much embarrassment.
Most of all dad, enjoy the time you have with your son. They don't stay nine forever, so savor these special times. Maybe you'll start a tradition that your son will continue with his own son. Who knows.....maybe they'll even let ol' gramps come along.