If all ribs are showing and the dog looks emaciated then you need to have your Vet diagnose what is wrong. There could be several causes including heart or whip worms, liver or kidney disease, metabolic such as over production of insulin or thyroid hormone. This is assuming the stool is normal. If not then digestive absorption disorders have to be considered.

It is not entirely unusual for a dog to leave behind 1/2 cup per 4 cups fed. Under normal circumstances a 4 cup diet is usually reserved for very large dogs. For example my previous dog maintained her weight at 74 lbs on 1.75 cups of dog food. If increased to 2 cups she would gain weight up to 80 lbs. I'm probably feeding both my 69 lb female, and 55 lb male about 2 cups of the same food as the previous one. Both are under 2 years of age. If I gave them 4 cups, they would pig out and look like pigs. The bottom line for your Boxer is food is not the issue. The fact he left some behind only means he isn't hungry. Thus the Vet's response is understandable per Jr. not eating, even though he did.

Rather than playing musical foods which might upset his intestinal tract, you need to consult with your Vet about the inability of the dog to gain weight. Pick a brand of dog food and stick with it, unless the dog develops a food allergy. If Jr ever catches onto the idea that if he fakes a hunger strike he's going to get human food, guess what he'll do?

For the record the dog food I've always fed my dogs, and they have thrived on it is the Propet brand. It has slightly more protein and fat for the performance formula than the name brands at almost half the cost. Even the ones I puppy sitted gulped it down without hesitation.

A comment about grass eating. Over 90% of the time a dog eats grass for one purpose- to purge or throw up. This is usually if not always a response to a build up in stomach acid. Splitting 1 meal into 2, usually solves the problem. Having said that my older female dog eats grass sometimes copiously without any side effects. I do believe she is part bovine.