Okay, I have a lot of girl friends and I tell them always, if you want to tell a boy something negative, like that you are not interested, ALWAYS keep it factual and not emotional. I might be wrong, but ... this is what I told one of my girl friends when a guy was really hard trying to flirt with her and was just, none stop trying.
First, say you are not interested. If he doesn't get the hint, tell him why you are not interested. If he still keeps trying, then you say, okay, I give you a chance, if you have these qualities, but first I need to see those qualities. Most guys give up if they are not emotionally interested.
But you have someone emotionally interested so that would be really hard. You are going to break his hart either way, so just do, and don't drag him along by giving hints and vague rejections. I will tell you, those hints and vague rejections is what hurts the most. Just tell him, and don't try to stick around like my ex is doing now.
Again, I might be wrong on all of this. All suggestions.