I’m sure others can chime in with specific book recommendations, but one very good approach is to simply answer questions your child has as they grow up about their bodies, where babies come from, etc. That’s an ongoing process that probably begins in toddlerhood.
I would imagine that the class in school, at least on the 6th grade level, will focus primarily on what changes to expect during puberty. You can inquire to find out more, I am sure. If the school is evasive and does not want to share information, then I definitely think that’s a sign to keep him out of the class. (I have heard horror stories about sex ed curricula, but I attended public schools and we didn’t actually discuss the “mechanics” of sex until eighth grade health, and even then it was “this is what your body is preparing you to do when you are an adult.” We also watched a video of human birth. Contraception wasn’t covered until high school health, and a student couldn’t take that course until 10th grade, and abstinence was definitely emphasized as the preferred course of action for anyone not willing to have a baby at that point in time.) Human reproduction was covered in biology class, too, but that was much more abstract and clinical.
I’d suggest that talks about sexuality come primarily from the same-sex parent once a child is middle school age and older, because really it’s all about how to be a man or how to be a woman. Hopefully you and your spouse are on the same page about that. There are resources like “Theology of the Body for Teens” that might be useful for you as well.