I have been in a similar situation. I once moved a little too fast into a relationship with someone I was better off being friends with. What I inevitably had to do was to tell him was exactly how I felt. Which was that he was a sweet guy and I loved being his friend, and that I preferred us as friends.
I’m sure before you asked me for advice, that you knew what must be done. There really is only one option. Just bring him down gently. Tell him that you think he is sweet as can be and that the time spent with him has been nice, but that you prefer being just friends. I’m sure you would agree that prolonging this confession would only increase the difficulty of doing so. So prepare yourself for the talk and do it as soon as possible. I only say this because it’s in your best interest, as well as his. I’d hate for you to stay in this relationship and feel uncomfortable or guilty, or for him to develop greater feelings for you. I wish the best of luck to you! Feel free to tell me how things go!