Thank you Gael,
First, I am glad to hear that those gums are pink and moist, as those tell us that his hydration is still good and there is no compromise to his oxygen uptake (thus internal bleeds, anemia, or lung/heart issues aren't likely here). As well, its good to hear that his belly is comfortable. Though if he does eat odd items, we need to be wary of a possible dietary indiscretion or blockage. As well, if he is warm, then infectious issues would also a concern at this stage.
With these in mind, we can try some home supportive care to see if we can allay his signs. To start, we’d want to see if his appetite loss due to GI upset and nausea. To try to rule this out or address it, we’d can consider treating with an OTC antacid like Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac). This medication of course shouldn’t be given without consulting your vet if he does have any pre-existing conditions or is on any other medications. Ideally, it should be given about 30 minutes before food to easy his upset stomach.
Once that is on board, we'd want to try to get him eat (as I am sure you have). Favorites are allowed or we can try to tempt him with a light/easily digestible diet. Examples of this would be rice with boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used in cases of gastroenteritis, notable Hill’s I/D or Royal Canin’s sensitivity.
Though if he cannot be tempted, we can consider starting syringe feeding. If it has been less then 24 hours, we don't tend to rush to this just yet, but I do want to mention it to you. And if we needed to, we can water down calorie rich diets (ie Hills A/D, Royal Canin Recovery diet, even canned puppy food) or use a liquid diet (ie Clinicare, Dogsure). As well, there are paste supplements (ie Nutrical) that can also be used. And these will all get nutrition in even if they won't take much. This way it would a means of getting food/fluids in, staving off dehydration and any risk of weight loss, and buying you time to uncover the reason for his poor appetite.
As well, do keep an eye on his water intake. If he won't drink water, then you can try to encourage him to drink low-sodium chicken broth. As well, wet foods (as mentioned above) are 35% water, so getting him to eat will help us deal with water intake as well. If he continues not to drink, you may wish to offer unflavored pedialyte via syringe feeding. While we cannot do this if they are vomiting, it may be an option for this situation. A typical maintenance rate for hydration in an animal 48mls per kilogram of weight a day. If you do give syringe pedialyte, this should obviously be divided up into multiple offerings through the day rather then all at once. This value will give you the total he needs for the day and is a good starting point to give you an idea of his daily requirement. If he does vomits if you give pedialyte, I would discontinue this as a therapy. (since we don’t want him vomiting because of our intervention).
Overall, if your lad wont' eat, feels warm, and is lethargic; we do need to tread with care. Foreign bodies could be a concern here but I suspect the warmth is a clue of a brewing infection. Therefore, we can start him with the above to see if we can settle him. Of course, if we do not see improvement over the next 12-24 hour, then we'd want to have his local vet involved. They can check his hydration, rule out fever, make sure there is nothing in his stomach that doesn't belong and confirm any infections present. Depending on their findings, the vet will be able to cover him with antibiotics and anti-nausea/vomiting medication and appetite stimulating medications by injection to help settle his stomach and get him back on track as quick as possible.
Please take care,
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