Please, guy's heart at risk?

I've been dating him for a month, he feels I'm the one. I don't have feelings for him and I need to break it off now before I hurt him. He knows that I didn't wnat anything serious. I talked to him from the beginning that I wanted to keep it light and fun. I thought my feelings would grow, but it hasn't. I know that "friends" is not what a guy wants to hear. Guys what do I tell him? Or atleast tell me what I shouldn't say.