I would not have a sit down. There are too many uncertainties about life: what's the point of bringing up something that may or may not affect him. It would be like discussion with your son (OR DAUGHTER) that there's a chance that they may be infertile. Part of life is dealing with its challenges as they present themselves, not worrying about them in advance.
That being said: If a parent notices that their son is losing hair at an early age and the son is visibly troubled by it, that might be a more appropriate time to discuss.
Just my thoughts. (From a guy who's dealt with both hair loss and infertility.