He needs to know that girls have periods, and what's up with all that, because they are having them in 5th and 6th grades. He also needs to know what sex is, and the funtionality of it, and that if he doesn't know what a term or word is that he can feel comfortable coming to you and asking what it means.
You would be amazed at what some of these kids, know, have seen, and personally experienced at that age. It's insane!!!

I was thinking more along the lines of how boys behave in middle school etc... and what to possibly expect. I have no idea.