You have to be firm and honest and don’t leave him with the impression that he might have a chance somewhere down the line.
Just say something like, “I’m sorry, I just don’t feel the same way about you. I value your friendship, but I don’t feel the spark of a boyfriend/girlfriend relationship.”
Be prepared that he might ask you another question like, “How do you know how you will feel about me in the future if we don’t go out on a date?”
Then you might have to say something like, “Don, I think you’re very sweet, but I’m just not interested in dating you.” You are basically re-iterating what you already said, but not having to explain it to him.
Then be prepared that me might say something like this, “What’s wrong with me? Everytime I like someone, they never like me back. Don’t you think you could possibly love me at some point down the line? Am I that hideous?”
Then you might have to respond with something like, “Don, you’re not hideous. You just can’t force someone to feel something that they don’t feel. We’re friends, but just friends, and I hope you’ll be ok with that. Now howzabout us going for a bowl of ice cream?”
If you regularly see, text or e-mail this fellow, try your best not to avoid him on purpose, or else he will really feel hurt. That is, unless he shouts something mean at you or acts excessively weepy or forlorn, then you can back off a little bit. Good luck.