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, ingestion of a foreign body or even part of a systemic disease.
I am going to make some recommendations below, if he continues to regurgitate the next step would be a hands on examination with family veterinarian and xrays as initial screening test. This is what I would try:
1- Do not give things like Pepto Bismol or imodium.
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 regurgitation, offer 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.
4- If he continues to vomit after doing the above he will need to be seen by your family veterinarian for a hands on examination and xrays.
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