Believe me, it took a lot more courage to call you up and ask for a date  then it will for you to pick up, answer…and quickly say no.  

Be nice.  Tell him, “I just don’t feel like we click romantically, but I would still like the opportunity to call you a friend if that is o.k. with you”.

Better to let him walk away with his head high, saying to himself, “maybe we don’t click…and later in conversation with his buddies complimenting you on how, “classy/nice/good you were in saying no”.  

Who knows, you might get asked out four years later by one of his friends that when asked about you, recalls what a nice gal you were when some girls choose the low road instead?  

And anyway…having a little good Karma never hurts anyone does it?