Have you tried leaving his lights off to see if he will actually go to sleep? When my torque went into partial brumation like this, I always encouraged him to fully sleep so he wasn't using up energy while he was off food.
If he opens hie eyes at all when lights are off, then he doesn't want to fully sleep, turn them back on. You can weight him once a week to watch for weight loss, some is normal during brumation.
Also, a vet visit to check for illness/parasites is always a good idea when they slow down like that, just to make sure.