What I am concerned about with your boy is that he is developing Hepatic Lipidosis(Fatty Liver Disease).
This is something that overweight cats are more prone to but that can affect any cat.
Let me explain...
If a cat decreases his food consumption by approximately 30% for as little as 2 weeks he is likely to develop hepatic lipidosis. Also, if a fat cat stops eating completely for 2-3 days he is likely to develop hepatic lipidosis.
Essentially what happens is the body goes into a state of starvation, and a signal is sent out that the body must mobilize the fat stores to provide energy.
Unfortunately, there doesn't seem to be a signal about how much fat to mobilize and fat cats have a lot... so it all gets sent to the liver to be converted from fat into glucose. And the liver gets overwhelmed and shuts down. This leads to nausea and vomiting, which means the cat won't eat, and the body tries to mobilize more fat. The cycle continues and the liver gets into more trouble.
Often, in older, overweight cats an underlying problem like cholangiohepatitis (an inflammation of the liver and bile ducts) could have been the initial reason for the loss of appetite. Or it could be something as simple as not liking his new food, a "cold," hairball or dental pain... but the consequences of not eating or eating very little are serious. With your boy this certainly sounds like it was all triggered by the food change!
I will add some links with further details:
Appetite stimulants can be helpful.
Cyproheptadine is one that I have used for years, but more recently I have started using a new appetite stimulant in cats that I have had great success with.
You could ask your veterinarian if he or she can get a hold of an antidepressant for humans called Mirtazapine (Remeron is the trade name in Canada).
Here is a link: http://en.wikipedia.org/wiki/Mirtazapine
It has been used as an appetite stimulant in cats and dogs for the last couple of years with great results. You would use it instead of the Periactin (cyproheptadine). Your vet could call the human pharmacy in Anchorage to see if they have it - they likely do! It is fantastic for cats as it only needs to be given twice a WEEK, so it is minimally stressful. Also, it is very effective and many cats eat ravenously within an hour of getting this medication.
And here is a link to cyproheptadine,the older appetite stimulant:
Now, as to what you can do at home for your cat...he definitely needs fluids, but what you can do is try to get some calories into him in a liquid form - that way he is getting nutrition at the same time as fluids. He HAS to get calories into him! This is so important!!
I suggest opening a can of tuna *in water* and offering the liquid.
Also, you can pick up Clam Juice in most grocery stores (sold in with the V8 or the canned tuna) and mix that with some water.
You could try Lactose Free milk (Lactaid is the Canadian brand).
Whiskas makes a tetra pack of "Kitty Milk" that is lactose free milk with flavoring added.
You could try getting some human baby food in meat flavors (check that there are no onions or garlic in the ingredients) and mix that with warm water and offer that, or syringe it in little bits into your cat's mouth. Beech Nut makes a line of baby food that has nothing but meat (beef, chicken, turkey or veal) in it.
If you cannot find this, you could find another meat baby food - just read the label carefully to be sure there are no onions, onion powder, garlic, or garlic powder in it.
Offer him some canned cat food, and mix it with water to make a slurry if he won't eat it.
Boil a chicken breast and then put it in the blender with water to make a baby-food consistency gruel to offer.
You could pick up nutristat http://www.agri-med.com/site/255063/product/NUTRST-4.25
It is a calorie-dense paste that you can syringe into them to get maximum caloric impact from a given volume of food.
Here is another link to ways to encourage cats to eat:
It has some good suggestions.
I'm afraid you may have to force feed him with a syringe. The human baby food (or making your own puree with cooked chicken breast in a blender with lots of water) goes through a syringe quite well.
I would like to see you getting at least a can a day of food into him - so about 3/4 cup of food. That seems like a lot! But if you can do it by giving him 10 mLs (2 teaspoons) every half hour that will make it easier. You really have to work on this!
The more he eats, the better his appetite. Every hour that goes by without him getting as many calories as he needs, is another hour in which his body is using fat and making the fatty liver disease worse. This is a very serious disease and cats unfortunately die from it! In many cases, we have to place a feeding tube into the cat's esophagus so food can be placed directly in the stomach.
So, please get very strict about getting food into him. You have to push it into his mouth with a syringe. If you cannot, then you need to see your vet for appetite stimulants or for a feeding tube. If he feels nauseated, he is not going to eat on his own. And the more he refuses food the more nauseated he will feel and the harder it will be to get him to eat!
You should have your boy seen by a vet as soon as possible as this really is serious and the sooner he is seen, the better his chance of survival!
Good luck with him, and please let me know what happens!
If you need more information, just click on reply and I will still be here to provide it!