Would you "just" kill your son if he wasn't healthy enough?
regarding your question, there are two different scenarios:
1- my son reaches the necessary neural development stage so that he is conscious, self-aware, interacts with other humans etc. then we can't kill him unless he requests it.
2- he has never reached that stage so he has not any personality, memory etc. then we can kill him. because there is not really a "son" here, not yet.
in fact, we humans continously kill pigs, dolphins and other animals that are more intelligent and conscious than a newborn.