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The most common cause of lethargy, vomiting and diarrhea in pets is a parasite problem. You should go ahead and see the vet to rule this out and go from there. I would also be concerned that he has an intestinal obstruction or other blockage.
To keep him from getting dehydrated, you can use plain pedialyte through the weekend. See the vet tomorrow for that fecal if you are able.
Let me know if you still have questions.