What do you tell kids about possible effects?

My son is almost a year OT and doing well.  I have been wondering when/if to tell him about possible long term effects.  He is 14.  I am talking about possible sterility, heart issues, secondary cancers....things that could come about later in life.  I don't want to worry him but would feel guilty if something comes up and I kinda was prepared but it came at him out of no where.  He knows he needs to be careful because of AVN and low bone density.  Of course, I think about the far future, he is married and wants to have kids---oops! doesn't work.  But then I think why worry him for nothing.  Why tell him and then have the possibility of risky behaviour.  He should know--right???  When and how?  What do your kids know?  I know it is only a small chance of sterility but this is the one that sticks with me.  At the beginning of treatment, I spoke with Onc about it and because my son was treated on a very high risk protocol with radiation--it is a real risk.   Secondary cancers and other damage can be checked for and  monitored and I feel these things can be brought up casually.  I guess the sterility is my big question.

Just curious,

Nana