My advice is don't get into a back and forth about why you don't want to date him. Just state the facts, but sugar coat it. Most creepy dudes don't know they are creepy, and don't bother being the one delivering that memo to them; it's too difficult a task, too hazardous and most importantly not your job. Let his creepy younger sister tell him he's a creep, let a town sheriff or Wendy Williams do the heavy lifting. Your job is to get this cat out of your life.
The next time he slithers your way and invites you to an all night Lord of the Rings marathon in his basement, look him dead in the eye and say, Hey Man, I dig your candor and it's very flattering and all, but I'm not interested in dating you, - a'right dude, be good.
See. Simple, yet convoluted at the same time. It's your basic F-off sandwich: Sweet. F-off. Sweet.
He'll walk way defeated, but strangely inflated. Get it?