In short, talk it out and come to some kind of agreement. But then, he sees a knockout. And when it was all over and everyone was happy. Period sex is one of those things. The way I've heard it is a guy, desperate to get laid, goes down to the docks to pick up a hooker. Is seems to be a taboo subject, like peeing after sex which luckily i now know.
It's not to say that your sex life should be meticulously planned and analyzed — after all, half the fun of sex is the expressiveness inherent in its spontaneity — but there are some things that are better discussed ahead of time instead of in the heat of the moment.
Any post asking for advice should be generic and not specific to your situation alone. Reminds me of a joke called Sandpaper Sally. Want to add to the discussion? At first, he thinks "oh shit, what now? However, actually having sex on your period all depends on how comfortable you both are with it. A good boyfriend will understand.