a. He's already grown up. The speaker really wanted to see him grow up, but for some reason it wasn't possible for them to witness the process. b. He hasn't grown up yet. The speaker really wants to see him grow up, but it may not be possible for them—depending on the circumstances—to see him turn into an adult.
a. is from the movie, "Tomorrow" and the situation is that the scientists are locked in some freezing ice and there is no possiblility that they can survive. And his son seems to be not grown up yet.