I would have a another sample tested just incase. As Graham has said, sometimes the sample can show no signs of the parasites and the animal still be infected. Tapeworm are one the worst for false negatives.

I will say that at this time of year they can sense the drop in barometric pressure and the change in seasons. You have to work harder to be certain the climate is ideal and photo period matches that of spring and summer. Otherwise they will try to aestivate. It is probably due to stress that he is acting the way he is. Keep him warm and moist, and keep offering food. If he begins to lose weight then you will need to force feed him.

Keep us posted.