You need to tell him that you have no feeling and want to stay as friend. Hurting him in this case is inevitable, he also need to experience the rejection in order to help him grow up. If you lie to him, he will hurt even more.
Then what's the problem there is nothing to think about it. Simply tell him you don't like him you are just friends. By telling him truth you are not hurting him. You don't like him then how can you date him when he is asking you. Tell him straight he'll understand this. Don't keep him wondering n waiting.