Most Helpful Guy
There is no way to do it without hurt, I hope you know that. Though you can do it, better than others. Many people just walk away, or say stupid stuff.
Direct is better. It makes it easier on everyone, even if it seems more hurtful. You just need to tell him that he is a nice guy, but the two of you are not right for one another. He may ask lots of questions or be surprised, but all you need to do is say "We have spoken about where we are, and what we need several times. Nothing is changing. It is apparent we aren't for each other." Then tell him thank you for understanding and end the conversation.
it is to the point and short. You aren't mean about it. He knows and has a reason, and understands that you are moving on and don't need "space" or "time" or whatever (and no one ever really understands what that means except that they hope there might be some sort of future after all).
Best wishes! P. S. do it in person not on phone or text. That is more respectful.