I have a casual relationship. We meet up roughly every couple of weeks, go for dinner and go back to mine or his. We have a good friendship and enjoy each other's company, but it's not serious and we don't really talk in between meeting up.
I usually cancel if my period is heavy but if my period is light/medium, I just use a menstrual sponge and get on with things. I've never mentioned it to my casual and he doesn't know. It's never leaked or been a problem. If he did ask why I never seem to have a period I'd probably tell him, but I don't think it matters. He doesn't ask, I don't tell.
I was talking to a friend about it and she said it's unfair to have sex with him whilst on my period without his consent. I don't think my period is any of his business and if it makes no difference, he doesn't need to know.
YABU - He should know I'm on my period
YANBU - It's not his business