Earlier quoted context omitted.
Exactly that. If it's some "policy" and not something objective you will sound clueless. Many people just sit in corporate environments waiting for the clock to hit 6, and do nothing until they leave. You don't want this kind of behavior. In case his work is time sensitive, it's a different problem.
That's the challenge I am facing. We are very flexible with the timings. It usually works for us. We seldom find people misusing this freedom. But then, once in a while we come across this situation. I am not sure asking the person to stretch a little more will help the coarse. I want him to be motivated intrinsically.
Have you thought that maybe leaving early is what motivates him?
Motivation is way more effective when it's something from inside (from the person) rather that from an outside stimuli (ie: a raise)
You need to understand what are his needs (what call him to action) in order to provide an environment that allows him to thrive