There is so much more to teach your 15+ year old son now that he is through puberty. You are preparing him for sexual maturity.
We want our sons to make informed decisions in the future and understand what those decisions might mean. For example, it is a misconception that a girl can only get pregnant one day of the month. There are actually several days when that can happen. You might think this is information that is too early for him to learn about when he is only 15 years old, but now is the time to get the information to him. Because he is not in a sexual relationship yet, it is simply biology and interesting for him. He will be more willing to learn it now.
Here is the question you want him to be able to answer. When is it most likely that a girl can become pregnant?
Illustration from I’m a Boy, Sexual Maturity
This illustration emphasizes that is possible that ovulation can take place on a few different days in a girl’s menstrual cycle and it might vary from one menstrual cycle to another. The sperm can survive for about four days in the fallopian tube to be ready to join with an egg.
It is likely that your son might not understand what a menstrual cycle is. I suggest you give him a copy of my book, I’m a Boy, How Are Girls Different? to learn about this important information.