I'm sorry to hear this may be a serious situation for your boy.
If he is ravenous, but losing weight, throwing up and is lethargic, these are classic symptoms of hyperthyroidism, which older cats are more prone to develop. I hope a thyroid panel was done at the same time as his routine blood work. Here is more information:
If no other testing, like imaging, i.e. an x-ray, MRI, CT, were done I don't know why your vet is rushing to do surgery. IF some sort of imaging WAS done and this led your vet to the conclusion that surgery is necessary to determine what the problem is, and either remove it or do a biopsy, etc., that makes sense.
There's always a chance it could be a benign mass or cyst and there's also a chance it could be a malignant tumor. Many times, it's very difficult to tell, without doing exploratory surgery. Ask your vet if there's any way he could do an abdominocentisis, which is where a very thin needle is passed into the abdomen to withdraw any fluid, or perhaps get cells from this mass, and that should reveal more information.
Even though all was well at his last exam, medical conditions can crop up quickly and must be dealt with quickly. Also discuss the possibility of hyperthyroidism with your vet.
I hope all will be well with your furry boy and please keep me posted on how he's doing. Thanks!
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