Whew, thanks to you both. Yes, he is an adult; he weighed about 600g at the end of February, and is a few years old now. I will take a weight on him tonight or tomorrow, and make sure he's not losing weight.
When you feed him, turn out the lights, leave the room, and come back in an hour or two.
Watching him with the lights on may be making him nervous as well.
The breeding thing is also a high possibility, as it is corn season. :]