How do I turn down this guy?

Well, last year he was my best guy friend. Then he moved to another state and came back for the summer. I figured out he has liked me for 2 years now and now he asked me out. One, he is leaving in like 10 days back to his state and I don't like him that way! It's hard cause he is very nice and polite, I just don't know what to do and how to say no nicely?!