Not really a plot hole, but look at the big picture of Christopher’s character: you have a character that has supposedly been trying to escape from this terrible life for twenty years with his son.
He wants to save his people from this hell. This character sees, firsthand, every day, how terrible people, or at least the people around him, are. Later in the story, he sees that said people dissect prawns with no regard for them. They’re treated worse than dogs. Finally, he is betrayed by the only person he’s ever trusted,Wickus, the person he promised to help. This person takes away his son.
And at the end, its not so much that Christopher goes with Wickus. That makes sense. It was his lines of dialogue where he says “I can’t leave you behind!” to Wikus, like Wikus is his best friend. Wikus’s betrayal did absolutely nothing to change the character of Christopher. Not. One. Thing. And at the time, Wikus not only thew him under the bus, he doomed every single Prawn on earth.
And none of those things affected the actions of Christopher’s character. That’s why I felt he was flat, and that’s why Christopher’s character was my biggest problem with the movie. He has no character arc and he doesn’t change at all from beginning to end, despite facing situations that should have changed him. He was a puppet.