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What color are his gums? They should be a nice medium pink color. Red or white is not normal and is an emergency.
Okay. So when I hear that a dog becomes suddenly lethargic, the first thing that comes to mind is a toxin of some sort and it is usually a human medication or marijuana. The next possibility is when some dogs have vomiting, they will retch so hard, their blood pressure will drop and they will become weak for a short period of time and then recover. The last thing that comes to mind is anaphylaxis or severe allergic reaction. Usually these dogs have very pale gums, but sometimes the gums are red. Their blood pressure will drop as well, but it doesn't recover and they start having bloody diarrhea and sometimes diarrhea.
If you are sure that he didn't get into any medications, I am worried about something much more severe such as anaphylaxis or possibly a seizure (I just thought of that).
Here is another possible cause- could he have gotten into any human artificial sweeteners? Can you read the ingredients in the pepperoni? Are there any sweeteners in them? Can you send the ingredients?