i am inclined to agree with VS that "i'm not attracted to you anymore" will hit him where it hurts unnecessarily.

you have to be clear that it's a breakup, without being curt. if he sees you as an asshole, then, well, you can't stop that. but it's better than you stringing him on. you just need to make it clear that you don't feel the same as you did and you don't want to be in the relationship anymore. he's a great guy, but you just aren't in it anymore.