I think like most of the guys here have been saying. You need to tell him as soon as possible. Don't give him a BS answer either. Be open and honest about how you feel. He will respect you much more for that, than if you just try and ignore him or act mean to him so that he will get over you.
If that's the way you feel about him, chances are that he probably feels like something isn't right as well. I don't think it will hit him out of the blue but at the end of the day you need to tell me ASAP for him sake and for yours.
If you do it in a kind manner, maybe you guys can be good friends afterwards?
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