First, I am glad to hear that his gums are normal. Though the reluctance to lay down makes be concerned about abdominal discomfort even if he is putting a brave face on when you press. With that in mind, our main concerns for all these signs would be severe bacterial or viral gastroenteritis, pancreatitis, or ingestion of harmful items (hopefully less likely at his age). As well, while not as likely to cause discomfort and this posturing, we can see vomiting in older dogs secondary to organ issues, metabolic disease and even cancer. So, we do need to tread with care here.
With this all in mind, to counter nausea and see if we can get his stomach more comfortable and him eating, we can treat with 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. Also if you give this and 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.
Once he is more settled, you can plan to tempt with small meals of a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, or scrambled eggs, or meat baby food (garlic/onion free only). 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. Canned pumpkin can be added to this to keep him fecal regular and just push any possible items he could have eaten (if you think he has). As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning slowly back to what you normally feed.
Since dehydration is a concern when our dogs are vomiting, we need to keep an eye on his hydration. To check this and make sure dehydration isn’t an issue, there are a few parameters you can test. Further to checking for gum moisture, you will want to make sure his eyes are not looking sunken and that he doesn’t have a "skin tent" when you lift the skin. To see how to check these, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you do see any of these signs already, then that would be our cue to have him seen before this becomes an additional issue (especially since its often dehydration that makes them feel unwell).
Overall, a wide range of agents could trigger the signs we are seeing. Even with his not showing pain, the sitting posture just makes me worried that he is hiding some discomfort there. So, we’d want to start supportive care to try to settle his stomach. Though if he cannot keep that or water down, appears dehydrated already, remains unhappy to lay down, 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, check bloods to ensure his organs are well, ensure nothing in his stomach that shouldn't be, or any sinister viruses present. Depending on their findings, his vet can treat with injectable anti-nausea medication, appetite stimulants, +/- antibiotics get him back feeling like himself.
Please take care,
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