"Just have a piece of candy and we'll talk about the next meal later." Su Jin took out a lollipop and popped it into Lu Mingxuan's mouth.
He wasn't intentionally starving Lu Mingxuan; it was just that they were still in a dangerous situation.
If he ate, he'd need to relieve himself... and they couldn't very well let him do that in his pants, or the car would be uninhabitable.
Untying him was too risky.