As some of the other commenters are suggesting, it may be too much of a jump to offer such a choice to the child at 18 if the child was not preparing for such an opportunity.
Homeschooling has its own critics and supporters and we don't need to get into that here. But my question is this: If he's going to trust his child to make a choice at 18, why not even earlier? How about 14? Or, in other words, why not have their education focus on learning how to make their own decisions as early as possible? And if school doesn't provide this for his children, should they be there?
Granted, the author doesn't outline what he plans on doing between now and the end of high school for his kids so maybe he is, in fact, preparing them for this decision. I just think that the decision would be too drastic to spring on an 18 year old who has had no preparation for it.