To keep an eye on teenager computer usage, what apps are there?

We have a 16-year old, you know. We're not always home. We want to make sure he's doing his homework when he's supposed to be and not playing video games when he's not supposed to be. I'd love suggestions for some way of keeping track of what he's doing on his computer when we're not around. If we could do it remotely that would be awesome too. He uses a MacBook Air.

Thank you!