GSD wont gain weight

Question
I have a two year old male GSD (neutered).  I am having a hard time keeping weight on him. When people visit or see him on a walk the firts remark made is on how thin he appears. He is a very picky eater so I try to mix some boiled chicken into his dry dog food.  I buy the "Exclusive" brand of dog food for large breeds.  So far I have had no success (checked for worms) and am concerned that my vet is missing something.  His current weight is 80lbs.

Answer
This looks like a pretty decent dog food. I didn't see a mention of how many kcals it offers but it looks ok. Your dog is still young, sometimes GSD's don't put on body weight until they are a bit older than two years old. However if he were mine I'd probably be concerned as well. Try adding some treats into his diet. Something that will put weight on quickly, like peanut butter which all dogs absolutely love. I think I would take him off the large breed for awhile as well, and put him on something with at least a higher fat content. The Exclusive chicken and rice performance has a higher protein and fat percentage and you may find he will gain weight using that particular food.  Some dogs just have a higher metabolism than others. We have one GSD pup who has been really tough to get weight on and keep weight on and our East German male who is VERY active is also difficult to keep weight one. We don't feed large breed to either one of them for that reason. Our two older females, who are three and four, on the other hand will get roly poly fat on the same food the other two wont' keep weight on with, so we DO feed them the lower protein , lower fat foods for large breeds.
Try that for six weeks or so and see what happens. If he still doesn't gain weight you might want to have your vet do some bloodwork to find out if you have a thyroid issue going on. Or pancreas. It happens but let's hope you just need a food change :)
Cindi