Take him aside and gently say "I am sorry if there has been a misunderstanding. I like you, but just as a friend. I am sorry if you thought there was more to it, but I just don't feel that way about you. I felt I should say something, I hope this doesn't hurt you. You are a (fill in positive adjectives of choice here) nice person, and I'd like to still be friends with you if we can, okay?"

See how that goes. It doesn't have to be at all confrontational. And if he does not still want to be friends, it will be sad, but try to be okay with that.