If he had human food and especially if it was off, that could certainly cause him a gastritis or pancreatitis here. Both of which could cause belly ache and discomfort. With this all in mind, we can try some home supportive care. 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),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.
Afterwards, we can plan to put him on small meals of a light 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 and will help with pancreatitis too. 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.
Overall, his signs do raise some concerns here. Therefore, we’d want to start the above supportive care now. However, if he doesn’t respond to the above within 12-24 hours; then we'd want to get his vet involved. They can assess him and treat with with gut safe pain relief, injectable anti-nausea medication +/- antibiotics to get him back feeling like himself.
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