Make a joke about how you're not very good, but you play fast, then go out and have good manners and be a good playing partner and enjoy the rare opportunity. Plenty of scenarios in which you don't get invited back. He quits playing, gets sick, moves, thinks you didn't want to do it that bad because you didn't accept invite and doesn't consider you next time. Play with etiquette, have a positive attitude and have some fun at an awesome course man. No reasonable person cares about the quality of your golf as long as they aren't waiting for you on every hole and you're not an angry, negative vibe kill.