Most Helpful Guy
You just tell him. Yes, he will be disappointed, but that's his problem, not yours. You are not responsible for his feelings or anyone else's. Just tell him clearly and firmly "Sorry, but I want to date guys closer to my own age." That should be good enough. If he continues to contact you after that, ignore him. It may seem rude but it's not. He's an adult and should be able to understand "no".