[carisear]

here's another angle: none of your business?

if his manager doesn't think anything of it, then it's not an issue.

if it bothers you that much, get a promotion, become him boss, and fire him. Otherwise, who cares what he does?

You said he does good work already, so to me it sounds like he's doing his job fine. As others have said, assign him more work if you have that power.

Although, it seems to me like you are better off quitting and finding a union or 9-5 job. Alot of employers now are allowing flex hours or work from anywhere type deals. If people not being at their desk at all times is important to you, look for a place that is specifically that type of job.