I'm against my son deciding to be child free.
Tell me more about why you're against your son's decision to not have a child.
I think he'd be a great father and I really want grandkids.
Even if you personally want to have grandchildren, it's wrong to try to push your son to have a child as you should respect the preferences of your family members. Not everyone wants to have children and that is a perfectly valid opinion to have.
I'm going to tell my son that he needs to have a child.
I don't think you should tell your son that he has to have a kid because that's inappropriate, and not your decision. He needs to decide for himself whether or not he wants to be a father.