16 is about average for losing virginity these days. Maybe its even younger. Whether you like it or not, there is no way you can stop it.
I believe in the talks about being too young, saving it for someone special, needing emotional maturity to handle that kind of relationship etc, but that's not the most important thing.
The most important thing to tell your son is "If you get that girl pregnant, I will cut your balls off." Buy him condoms and tell him if he ever gets to the point of having sex, he MUST use a condom every single time. Tell him every day, do NOT get that girl pregnant. You do not want that nightmare of a pregnant teenage girl making plans to make you a grandma.