Most Helpful Girl
Why can't you just tell him that?
It's better to be honest so he will stop pining and move on to someone that might accept is advances.
There isn't a "nice" way to reject someone. It's going to hurt either way. The best thing you can do is be honest and explain your reasoning behind it in the nicest way possible. Don't degrade him in any way. You could even tell him that he is a really nice guy and he will make some other girl very happy!
Honesty is the best thing you can give him.