How do I make him stop asking me out?

I've had the same problem. Constantly being harassed by your male friends can be a very hard thing. Maybe you should just really, and I mean REALLY lay it on the line (in a nice-ish) way, that you're not into him like more than a friend and that you'd really appreciate it if he would just let your friendship stay a friendship because his constant pestering is weakening it. Of course, not in those words, but make your own for it, and try to be as gentle as possible because you don't want to lose him. Good luck!