At 9.5 he knows a lot more than you give him credit for knowing.
There is no such 1 time "talk"... it is communication that starts when children are very young asking questions and answering them appropriately per the age of the child.
I suggest open communication and being honest because wouldn't you prefer he get factual information from you than what is talked about at recess and at school.
If you are uncomfortable, talk while you are driving or walking. I like the idea of getting biology books from the library as well so you can be factual.
At our elementary school, the school nurse has separate meeting with the boys and with the girls starting in the 4th grade about body changes and what to expect and on. Sadly, this is the only information some children get because their parents do not communicate with them.
Kudos to you for wanting to talk.... keep those lines of communication open... it is a long road through school and he needs you or someone else he trusts to come to with questions and concerns.
Good luck