A teacher's frustration

One of the biggest frustrations a teacher can experience is a student with great potential who doesn't want to even try to live up to it. That's happening to me at this time.

S is a student in one of my in-company classes, he's 35 years old, reads/writes/speaks English quite well and I've seen heaps of potential in him since the first day of class. He'll be taking the BEC Vantage exam on 4 December 2004, and will surely pass with a 'C' grade (the minimum required to pass). He hasn't done a lick of studying or homework and has skipped many lessons, but is still good enough to pass. I've been trying to encourage him to live up to his potential since our lessons started in January, but it hasn't worked. He continues on as before.

If he studied, wrote essays for me to correct and attended class, he could surely pass with the highest grade possible. However, that doesn't interest him. He just wants to get by.

My feelings of frustration are somewhat placated knowing that I have made a 100% effort to try to get him to study (and will continue to do so for as long as he's my student), but I am still quite disappointed that he is refusing to try.

What more can I do?