How to tell him I don't like him.?

Well, the best way to put him down would be, "Hello _______(Name) It seems like you're interested in me, but honestly, I don't feel that way. I am really sorry and I hope you aren't hurt. I just don't think that this would really work out. I'm really sorry, but I'd just rather stay friends." (Or something along those lines) Hope I helped! :D