If you don't want to be with the guy, tell him so. You don't mean to string it out, I'm sure, but it sounds like he's going to keep on asking until he knows for sure.
No need to be harsh about it, just instead of changing the topic... say no.
Ok say to him, that you care to much about your friendship, to sacrifice that by going out. it does work.
I know this sounds lame, but it lets him down, he still thinks you care about him & makes him feel somewhat special, I know that was one of my concerns when going out with a mate.
Be like sorry but no. I want to become friends first. And when he askes to do something do it. Like really be his friend. Don't say it and never do it.
Be straight forward in your approach.
Tell him how you feel about him
Thank him for offering to take you out, but tell him that you are not interested.
Dont postpone this or avoid confronting it.
He will get disillusioned and go away
Definitely be straight forward about it. Makes it easier for both of you. Dragging it out makes him think he has a chance and you will have to put up with it.
You should accept and maybe know him better. Who knows? You two may make a great couple!