I know that part of the 5th grade curriculum here talks about physical changes but doesn't get into sex ed per se. But any kid in middle school will hear info,right or wrong, in school, on the bus, in the cafeteria, at recess,etc etc. so my advice would be to just ask him if there is anything he would like to know,perhaps a book geared toward kids his age at the library may help. I would bring it up during an activity like driving in the car with him for example so he doesn't feel cornered or on the spot,y'know? Good luck!

Heidi