Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner, but wanted to touch base with you about your lad.
Based on his current signs, it sounds like he is still nauseous even if he isn't vomiting today. And it is that nausea that will be putting him off his food. Therefore, we'd want to try to soothe his stomach while keeping a close eye on him. Just to note,possible causes for what we are seeing would be bacterial or viral gastroenteritis, pancreatitis, parasites/protozoa infections, dietary indiscretion, and harmful ingestion (hopefully less likely here).
With this all in mind, we can try some home supportive care to try to settle his stomach. To start, you can consider treating him an OTC pet safe antacid [ie 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]. 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. Though if he cannot keep it down due to nausea, that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Afterwards, you can consider starting an easily digestible diet like cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). 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. As long as improvement is being seen, I usually advise continuing this until signs are settled, and then weaning slowly back to your normal diet.
Or if he cannot be tempted but isn't actively vomiting, we can offer or syringe feed watered down canned puppy food, Hill's A/D, Royal Canin Recovery, or Clinicare Canine/Feline Liquid Diet. All of these are critical care diets that are calorically dense, so a little goes a long way nutrition-wise
Overall, his signs do raise some concerns here. Therefore, we’d want to start supportive care to settle his stomach. If he cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above within 12-24 hours; then we'd want to get his vet involved. They can assess his hydration, rule out fever, 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 feeling like himself.
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