I was in the same boat ;)
Here is what I did...
There was this guy I sorta liked, but he was a good friend of mine. I didn't like him enough to date him, so you and I differ in that. Well, I casually told him I liked him.
By casually, I mean you should bring it up in a conversation, almost like it doesn't faze you. Bringing it up casually would be saying, for example, "that is why I like you." If he says he just wants to be your friend, don't fight him or you will lose a friend. If he only sees you as a friend tell him you told him out of respect because you feel the same :p