Personal question... & periods...?

I just got out of a long term relationship, and in it we were completely comfortable engaging in while I was on my period. But now, I've recently started up a "friends with benefits" type relationship with someone I know (don't hate, most people have tried it at least once in their lifetime) and it's going really well to say the least. We've hooked up about 4 times so far, and yesterday he called, wanting to meet up with me tomorrow evening. One problem...dear old Aunt Flo decided to pop in for a visit tonight. What should I do? When he calls to confirm plans with me tomorrow, should I tell him then or wait until the moment comes? Honestly, this is going to sound pathetic but I really don't want to risk losing our plans to meet up, because even when we aren't having I love spending time with him. I've never been so physically attracted to someone on every level. So maybe you can see the dilemma I'm in. No rude comments PLEASE! I don't care to hear them!