Whats wrong with my German Shepherd?

I have a 10 year old German Shepherd. A couple weeks ago we also had a pomeranian. They didn't get along very well but he dealt with him. We had the pomeranian for about 4 years when he suddenly got out of the house at night and never came back. He was very old and we think he left to die. This happened probably 2-3 weeks ago. Around 1-2 weeks ago my German Shepherd stop eating. He would go every now and then to eat some on his dry food. He soon left it there for a couple days. We started feeding him wet food but he would just pick at it but not even finish 1 little serving of it. Now he will not eat anything. We have tried going back to his old food, wet food, and human food. We've tried pepperoni, sausage, cheese, etc. He still drinks his water. He does NOT throw up or have diarrhea. He seemed to slow down a little like he has no energy. He has no desire for food or to go outside and play. He sits in his favorite spot on the coach next to my mother all day. He has a sad look on his face 24/7. His nose is wet and cold, his hair is the same nothing has really changed. HELP ME! We absolutely love this dog and do not want to think about him passing, we all tear up thinking about it. He had an enlarged prostate a couple years ago and it would cause dripping of blood, but he was given medication and it hasn't happened since. Does anyone have anything? Please help it would mean a lot to him and my family. Thank You.