"... so I'm wondering if I can check the computer he's using as well."

it can be pretty tricky to dig deep if you are not an expert, and Windows/Internet Explorer keeps a log in some pretty deeply hidden files (I posted something about it a couple of months ago).....but IF you are authorized to do investigative work for your office, there are ways. (I would not recommend messing with someone else's computer otherwise)

There are 'stealth' programs that can be installed that will record almost any level of activity, right down to keystrokes. I don't like the idea much, as it can so easily be abused, but I suppose it can be a tool when there is serious need.

If you think your office NEEDS something like this, try Parents Friend