I would just say thanks, but no thanks. If you start giving reasons or explaining your thoughts, he will push an emotional crowbar into the cracks in your arguments and you might be left either feeling like you’re now obligated to go out with him (regardless of whether that happens, or not, you’re never obligated to date anyone! You don’t owe people relationships), or wondering what else you can use to put him off. “No” is a complete sentence!