How do I tell someone that I'm really not interested in them?

I've known this guy for about a year and a half now. I'm 19 and he's 25. Truthfully I wouldn't even consider us friends. He always tries talking to me on facebook and tries getting me to do things with him like go out to dinner, play tennis with him (I do play tennis, but I just don't wanna play with him), hang out at his apartment. I know he's probably interested in me because I know he's desperate for a girlfriend since he's been single for quite awhile. I'm not interested in him like that. How do I tell him that I'm not interested in him without hurting him?? And I know yea I'm probably gonna hurt him a little bit but I don't wanna come off as mean or anything like that. I have to do this because whenever he asks me to do something I always say "no, i'm busy" or "i just don't feel up to it right now". I just wanna get it done and over with, if I don't he's gonna keep thinking that he still has a chance with me.