How do i tell him i can't have with him?

Me and my boyfriend are ually active and he's been looking forward to seeing me all week and will want to have (hes a boy lol) But i'm going to be on my period. How do i let him know? Would it be a complete turn off for anything else if i just told him out right or should i just subtly say 'i cant' if he puts his hands down there? Or should i let him know beforehand so he doesn't get his hopes up and be let down when it comes to it? cos he will...