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Sorry to hear this. The fact that he is not even keeping any water down and that this has been going on for several days and is now lethargic is very concerning. This suggests that he is becoming dehydrated. I am also concerned about the underlying cause of the vomiting. A gastrointestinal obstruction would be one consideration, severe gastritis or pancreatitis would be others.
He certainly needs to be seen by a vet today, a blood test and abdominal imaging may be recommended. He may need to go on a drip and have antisickness medications administered.
I hope that answers your question? Please let me know if you need any further help or advice.