Friend, vomiting, diarrhea and lethargy can be caused by many different medical conditions. It will be difficult for me to determine the exact underlying cause without a hands-on examination and some testing. But, common causes are dietary indiscretion, intestinal parasites, ingestion of a foreign body, infections, etc. to name a few causes. To determine the exact underlying cause he will need a hands-on examination by a veterinarian, blood work, fecal analysis and x-rays as an initial screening test.
I am a bit concern if his gums are pale; this could be an indication of anemia, poor circulation, blood shifting to other organs, etc. If you have access to ER vet I would have him checked tonight. If you do not have access to ER vet this is what you can do until he is seen by your vet in the morning:
1- Do not give things like Pepto Bismol or Imodium. Start him on Pepcid AC (famotidine) 10mg 1/4 tablet 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 6-8 hours. Need to give his gastrointestinal tract time to rest.
3- After 6-8 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.
4- Follow up with your family veterinarian for a hands on examination in the morning.
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