Sometimes they don't come out for a week or two. Usually it's because they're in shed, but new animals do it too. If he keeps it up for over a month AND doesn't eat, it might be worth going to the vet for a fecal smear to check for parasites.

Usually, though, they're just nocturnal. Or "crepuscular?" They come out after you turn the lights out and bop around for a couple hours, "hunting" or just exploring. So next time you turn the lights out, wait a half hour, and then sneak back in with a flashlight. Unless he's in shed, there's a good chance he'll be out.