I'll second this. The issue of whether you are paying for his time or his performance is a cloudy one. The paying for time argument invariably gets into issues like how much other unproductive time you spend doing things like talking to coworkers, using the bathroom, staring out the window, and browsing the web. In general it seems to degenerate into the horribly inflexible clock-out-when-you-go-to-the-bathroom factory-ish policy.

So if you are happy with his actual concrete performance, then I'd say don't worry about it. If he can make you happy spending 15 minutes of real work per day, then you need to think hard about how to better leverage his talents. Unfortunately, increasing your expectations of him while keeping everyone else the same will not make him happy either. It's still a tough issue.