I mentioned this in a thread earlier today, but you may want to check out this book. It's meant for parents of kids with Down syndrome, but I think it would work well for autistics too. Lots of diagrams, and the text is very straightforward. Just make sure you read it beforehand so you can show him the parts he needs to know. (No reason to get caught up in the details of menstruation or anything else irrelevant.)
https://www.amazon.com/Teaching-Children-Syndrome-Boundaries-Sexuality/dp/189062733X
This is a good resource. It is appropriate for his age.
There is a book from woodbine house. I don’t know if it would help exactly but it is a good resource.
https://www.amazon.com/Teaching-Children-Syndrome-Boundaries-Sexuality/dp/189062733X