Not a guy but have a masters degree in child development and raising 12 yr old boy/girl twins.

There should not be just one talk. Sex and sexuality are all around you all the time. It needs to be presented as interwoven into your daily life.

If you want to have "a talk" i suggest starting from a biological perspective complete with anatomy pictures. Then moving to an emotional daily matience type talk. Then onto relationships and sexual well being then moving onto risks like disease and protection. Then onto respect. And then outlier things like lgbtq healthy porn watching habits (not training yourself on weird shit as a kid) , lifelong changes, womens bodies and child bearing, menopause, periods, anatomy, emotional health, breast feeding, how not to be a jerk or creepy or harassing or rapey. How to date. How all these things are normal weving back into biology. Its important to show that you are comfortable with him asking questions and that it is all normal and healthy. Its also important to talk to him about rape/molestation and safety for himself and that its ok to tell someone if he feels uncomfortable or if his friends are struggling. Talk about body image and self care. Talk to him about changes in puberty and how embarrassing and awkward it is but hell grow into it. Present this in a positive light and emphasize good loving relationships over hook ups or porn.