What you are describing is known as "acute abdomen". This is a result of visceral pain, which has a long list of possible causes. These include pancreatitis, dietary indiscretion or garbage gut, hemorrhagic gastroenteritis, foreign objects, obstruction, and torsions.
Since he is still showing signs of pain and lethargy, I would start by repeating the CPL snap. Being a Yorkie, I'm definitely suspicious of pancreatitis.
If this is still not positive, I recommend continuing diagnostics with an ultrasound of his abdomen. That can be very helpful in distinguishing causes of acute abdomen.
Even without a diagnosis, I recommend getting him into the hospital for supportive care. I would start IV fluid therapy, anti emetic injections (anti vomit drug called Cerenia), and eventually appetite stimulants and a bland diet.
I hope that helps.
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