New dog stressed and wont eat?

i just got a 13 month old GSD he was imported from germany when he was a baby, by a breeder. She just recently had his hips and elbows checked and he didnt p with one elbow, so she cant breed him. she is a family friend, who wanted to place he dog who she greatly cared for in a good home. so she gave him to me.... he had been with me for a week and i am having a very hard time getting him to eat. he doesnt seem to be settling in. first off he is extremly fussy when it comes to food, he old owner said that he was a really good eatter when she had him, i have some of his old food be he wont even look at it when i feed it to him. he was loosing a little weight before i got him. so his old owner took him to the vets an had a complete workup done on him. everything was fine. so in the last week he hasnt hardly eatten anything, he has lost a lot more weight, he is probley 20 lbs underweight. i was hoping that maybe somebody had experience with something like this and could help me figure out how to get him to eat and put on some weight. are there some supplements for weight gain that i can give him. I have tried many different kinds of food from cheep grocrey store bands to high quality expencive foods, None seem to do the trick. he wont even eat canned food. a friend of mine told me i should try a calming supplement to help him cope with the stress of moving, can anyone suggest a good one? Thanks for the help!