Most Helpful Guy
You need to take ownership of yourself, and of you body. Just because you planned to have sex, does not mean you can't just tell him you changed your mind, and you are not ready. Even if you planned it for a year. If you tell him, and he doesn't understand, then you are lucky that you are not loosing your virginity to a jerk. Any decent guy would be understanding of your situation, and would want to make you comfortable and wait, or even not have sex at all. Yes, a guy does look with anticipation when he thinks he will have sex. But unless he's a jerk, that will not matter much at all to him.
Most Helpful Girl
I would personally be very very honest with him. Call/text him and say that you’re really sorry and although you thought you were ready you don’t think so anymore. Say that you truly thought so before, but it has changed and you hope he forgives you as it’s so last minute. If you try to use some stupid excuse he will probably just try to reschedule and then you’ll still have the same issue of getting out of it. If you STILL wanna go for the excuse although it wouldn’t really work, you could just say you got your period, enough said