My cat Cosmo is very.often sick especially after eating. He

I'm sorry to hear Cosmo is experiencing this issue which must be stressful for both you and him.
There are a wide number of conditions that can cause these symptoms including gastritis, IBD, food allergies, cancers etc. and we often need to run several tests to get to the bottom of things.
You say he has pancreatitis; do you mean he had a bout of pancreatitis in the past or that he is currently suffering with an inflamed pancreas? If he currently had pancreatitis, he likely needs pain relief, anti-nausea medicine and perhaps hospital admission for fluids and supportive care. Pancreatitis is a very serious illness indeed and requires quite intensive treatment.
When the vet examined him today they will be ensuring his weight is steady, checking his hydration, assessing for any abdominal pain, palpation his abdominal organs and intestinal tract for any irregularities, checking if he has a fever etc.
Test wise, the first step would be a general blood test (which should include full biochemistry, hematology, electrolytes, tli/cobalamin/folate). We may get our answer from this or it may simply direct our next investigations.
Usually, we will want to perform an abdominal ultrasound to examine the internal organs and, often, an upper GI endoscope is useful to look inside the stomach and upper intestines. Not uncommonly, we will take little biopsies when examining the intestinal tract.
You mention diet and with cases like these it is always useful to trial a strict 8 week hypoallergenic diet- this means a prescription hypoallergenic food such as Royal Canin Anallergenic and Nothing else (no nibbles of the other cats food, no treats etc) for the entire time.
It's worth saying, if we suspect that he is regurgitating (bringing back undigested food) rather than vomiting (forceful contractions often associated with nausea), we would likely also want to perform an xray to assess for a condition known as mega oesophagus.
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