What is the normal body temperature of dogs?

The normal body temperature for a dog is 101 F or 38.3 C. It is great that he is still eating and finishing all his food, as usually one of the first signs of illness is a complete disinterest in food. Dobermans don't have much of a coat to keep them warm, so make sure during Winter he is kept nice and warm. It could be just because it is Winter and that it is cold that he sleeps more, and if he is sleeping more and not running around as much, naturally he won't be as hungry. It is obvious you love and care for your dog very much, I am sure he is quite ok and hasn't got a cold as it is not that common for dogs to get colds.