I am sorry your boy is not feeling well.
There are a lot of possibilities with a 15 year old cat. The first is an abscessed tooth or root in the gum. The next is a liver or kidney issue.
He should not get dry food as it is too rough for him.He needs caned or baby food.
He should get subq fluids to hydrate him.
Although overweight can cause diabetes and other health problems, it is very dangerous for a cat to suddenly lose weight. That is because a cats liver becomes accustomed to a certain caloric intake. If that amount is cut (for any reason) the liver will think it is starving and grab protein and fat from the cats muscles and organs. This can cause liver and organ failure, muscle wasting and lead to a weakening of the immunity and a disorder called 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 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.
See if he will drink tuna water or milk. Offer tuna,, baby food., a/d cat food, chicken or fish. The smell of food cooking awakens the appetite sensor in the brain
human baby food in meat flavours (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. Here's a link: http://www.beechnut.com/Our%20Baby%20Food/Results.asp 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.
Cats that eat dry can have bouts of dehyration.Indigestion is caused by inadequate chewing. Vomiting is very common. When acid reflux builds up in the stomach, they begin to vomit. Even if they have always eaten this,, they often become intolerant to corn and gluten which are used as filers in most supermarket cat foods.Vets push science diet because they sell it but it is not a great food as it too contains corn and gluten.
Lack of fluid in the bowel also leads to vomiting, Ingested hair can also be a factor.Anorexia for any extended time leads to liver damage and causes nausea and vomiting as well.
I would add some canned or baby food for him
Serve the two foods separately - do not mix them - add a half pepsid a/c tablet, crushed up into the wet food one time a day.
Put hairball remedy or even plain vaseline on his paw for himto lick off as well.
If his tummy needs a rest, feed baby food with no onion or garlic for a few days.
If he wont eat, and your cat is not diabetic begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Give him droppers of unflavored pedialyte a few times a day to make sure he stays hydrated. GO VERY SLOW SO HE CAN RETAIN IT.
If he throws it up have him seen immedately.
Please let me know how he is doing. I will always be here for you.
I hope he feels better very soon.
Sincerest best wishes,