First, I would note that I do suspect your lad is uncomfortable given then elevated breathing rate without change to the gum color/perfusion and the fact he isn't lying down (often they do this to avoid putting pressure on their bellies). As well, while we cannot exclude diarrhea or excessive protein loss in the urine as the cause for weight loss, his progressive appetite loss certainly could be playing a major role here. And the less he eats, the less he will need to pass as stool and less energy he will have. So, those signs are all interlinked.
With that all in mind, potential causes for this in Schmidt's case would be a potential foreign body, harmful ingestion, pancreatitis, or severe gastroenteritis. As well, if we did have diarrhea or urine protein losses, then other bacterial/viral agents, parasites, and even organ compromise would have to be considered.
Now with this having gone on for weeks and his being at a point where he is refusing to eat at all, we'd be best to have him seen. That way his vet can pinpoint which of these concerns is present and start injectable anti-nausea medication, gut safe pain relief, fluids, and appetite stimulants.
Otherwise, if there is any delay in having him seen and we can at least try some supportive care.To start, you can consider treating him with an antacid. Common OTC pet safe options would be Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac), or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned.
Aftewards, we'd want to tempt him to eat. You can try favorites or a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, cottage cheese, or scrambled eggs (made with water and not milk). The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset.
Though if he cannot be tempted, then we'd need to think about syringe feeding with a calorie rich diet like Hill's A/D, Royal Canin Recovery or even canned puppy food. As well, there are also liquid diets like Clinicare Canine/Feline Liquid Diet or Dogsure. All of these are nutrient dense, so a little goes a long way nutrition-wise. And these could just help get some more calories in and halt weight loss even if we can’t get a huge amount in.
Overall, there are a wide range of agents could trigger the signs we are seeing. Therefore, we’d want to start supportive care to settle his stomach to see if we can get him eating. Though if he refuses and since this sounds advanced already, we'd want to get his vet involved. They can assess his hydration, rule out toxins, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with injectable anti-nausea medication, appetite stimulants, +/- antibiotics to settle his stomach, and get him back settled so we can get weight back onto him.
Please take care,
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