I wouldn't want my kids to be lacking knowledge or worse, having the wrong impression about human sexuality as they grow and explore. So at a decent age to be determined as it comes, I'll explain to him starting with biological evolution and how our species, like most mammals and any creature for that matter, exist as the two genders and what got split (chromosomes) and is then combined between male and female at intercourse to produce a new living being for the continuity of the species.
Kinda flat explained like that in a few sentences, but I'd include some humor and be sure to give them the right impressions about not being ashamed of sexuality and it being natural and most important, can COME TO ME WITH QUESTIONS! lol. An no, I wouldn't be doing any 'save it for someone special' talks or unrealistic expectations. While I would explain how babies are made, I'd also be sure to explain the dire seriousness of that, and horrid diseases hiding in our blood and fluids, and how to go about having his 'fun' in both safe and baby-free manners. Then buy him a box of prophylactics
Though if I have a daughter, then I'm investing in an adamantium chastity belt!
j/k.j/k lol