Friend, when we see vomiting of undigested food it is called "regurgitation" as the food has not been digested. Possible causes could be esophagus disorders (megaesophagus, strictures, parasites), gastritis, etc. to name a few causes.
I am going to make some recommendations that you can try at home if he continues to regurgitate the next step is a hands-on examination with your vet and x-rays as an initial screening test.
1- Do not give things like Pepto Bismol or Imodium. Start him on Pepcid AC (famotidine) 10mg 1.5 tablets every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 8-12 hours. Need to give his gastrointestinal tract time to rest.
3- After 8-12 hours offer a small amount of water. Wait half hour, if no vomiting, offer a small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day. Keep on this diet for several days then wean into regular diet.
Note: When you feed him put his eating dish on top of a small stool to force him to raise his head when he is eating.
4- If he continues to regurgitate after doing the above he will need to be seen by your family veterinarian for a hands on examination and x-rays.
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