Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Sugar has a recent history of frequent vomiting as well an initial increase in appetite that has now decreased. I think that you need more information before you make any permanent decisions.
Vomiting can be related to something as simple as a quick change in diet or treats, eating dry food too fast which leads to over eating, expansion in the stomach and vomiting, getting into something he should not have like the garbage, a bug, plant material or a toxin.
More serious causes are chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, heartworm disease, internal organ failure, hyperthyroidism, a full or partial gastrointestinal obstruction or even infiltrative cancers like lymphoma.
Because these vomiting episodes have been recurrent checking some bloodwork was a great idea.
ALT or alanine aminotransferase is one liver enzyme. It can be elevated due to cellular leakage and can indicate primary or secondary liver disease. That means that his liver is irritated and leaking enzymes, but it can be due to priXXXXX XXXXXver disease or any disease in the abdomen that causes irritation or inflammation of the liver, including breaking down fats for energy to live when the kitty isn't eating well.
The complete blood count results you reported are actually fairly common in a stressed or sick animal and don't point to anything specific.
Was a T-4 checked? Hyperthyroidism and secondary organ damage from an overactive thyroid could certainly lead to what you are seeing with Sugar.
Depending upon exam finding abdominal radiographs may be needed too. If those look normal then a heartworm test, as vomiting is strangely one of the most common signs of heartworm disease in cats, should be done as well as a specific blood test for pancreatitis, a fel spec PL (feline specific pancreatic lipase). This test is highly specific for pancreatitis in cats.
The next step diagnostically after those blood tests is an abdominal ultrasound and/or either exploratory surgery or endoscopy to gather biopsies of his gastrointestinal tract to look for inflammatory bowel disease or cancer.
To start now you can try giving him an acid reducer to see if that helps. You can try either:
1) Pepcid ac (famotodine) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 12 to 24 hours
2) Prilosec (omeprazole) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 24 hours.
These are very safe and can be used long term if necessary.
Dietary changes may help as well.
For now you can try a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If this helps him there are some long term diet changes you can make that will help.
You could try feeding a canned only sensitive diet stomach. This will stop him from eating too much dry and having it expand in his stomach and also is easier on his stomach then regular foods. Royal Canin and Hills both make sensitive stomach foods. When you start to convert him from the bland diet to his new diet make sure to do so slowly, a little more new and less bland at each feeding. It should take 5 to 7 days to switch diets.
If he is still vomiting intermittently you can try more restrictive diets such as prescription Hills i/d or Purina Veterinary Diets EN. If these aren't enough then perhaps a true hypoallergenic diet such as Hills z/d or Royal Canin Duck and Green Pea will work.
In short the lab results you are reporting tell us that the liver is affected but not much more then that and only tell us that ideally more testing should be done.
Best of luck with your fellow, please let me know if you have any further questions.