Either way you look at it, your 15-year friendship has been drastically changed, probably permanently.
So, if you go on a date, and you kiss, it's going to probably be very awkward for you because you're kissing someone whom you've had no romantic/sexual feelings for. For him, it's been what he's dreamed about for 15 years. If it doesn't work, it could end a friendship.
If you don't go on a date, if could end the friendship. That's 15 years of feelings that you're rejecting. Neither of you will look at each other the same way. You probably wouldn't innocently put your arm around your friend or give him a hug without thinking twice and making sure not to lead him on. Anytime he looks at you in the eye, your paranoia might kick in and wonder if he's feeling romance for you again.
In many ways, his friendship with you may have been based on his love for you. So, if you reject his date invitation, then his entire reason for being your friend doesn't exist anymore, and he'll probably phase himself out of your life.
It's a very tough position to be in and personally, I don't envy you. What might help is to have a very clear and frank discussion, and obtain a strong commitment that you will both make sure that your friendship will still go on whether you agree to go on a date with him or not.
Best of luck.