[aquark]

You say that he has "no real set hours, just that he gets stuff done": is he getting stuff done?

If he isn't getting the expected stuff done, then that is the discussion you need to have.

If you aren't happy with the amount of stuff you are expecting then discuss that.

Some days are productive and some days aren't (and I find unproductive days tend to follow really productive ones). Try and view it purely from an output point of view, and not what his method is.