Friend, lethargy is a very non-specific symptom and can be related to many different medical conditions that affect dogs. The fact that he vomited last night we need to rule out things like: Dietary indiscretion, ingestion of a foreign body, pancreatitis, etc. to name a few causes.
Unfortunately we are very limited as to what you can do at home. I would recommend you consider having him checked today by ER veterinarian for a hands on examination, blood work, supportive care and +/-xrays as initial screening test. Treatment will be based on exam and test results. If you do not have access to ER veterinarian this is what you can do until he is seen by family vet:
1- Do not give things like Pepto Bismol or imodium. Start him on Pepcid AC (famotidine) 10mg 1/8th of a tablet every 12 hours. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 2-3 hours. Need to give his gastrointestinal tract time to rest.
3- After 2-3 hours offer a small amount of water. Wait half hour, if no vomiting, offer small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day. If possible blend the food and feed him small amount with a dropper.
4- Follow up with your family veterinarian for a hands on examination.
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