To fatten up dogs, we always use raw lamb mince (I think Americans call it ground?) Lamb has a high fat content and can be given alone or mixed in with kibble. Otherwise look for kibble that targets weight increase.
Also, keep an him, if he is not putting on weight despite increasing the volume of food, consider getting him tested for medical issues such as EPI, where they eat and eat and basically starve to death at the same time because they can't digest food.
if your GSD is eating and eating and eating and losing weight - please consider testing for EPI.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.