Hi, first I would ask if your cat has been vaccinated against the usual diseases like feline distemper, and of course Rabies? Also if your cat is an inside outside cat, has he had a Leukemia vaccine? Like Savas says, there are lots of things that can cause the symptoms he is showing. What concerns me is the fact he has pale gums and is very lethargic and of course losing weight.
You defintly need to have him checked out by a vet. If it is not serious and perhaps a infection from getting in a fight, ( abcess etc, can be undetected under the skin,) antibiotics can help. If he is very sick, he may need an IV and of course to be left in the hospital for care that you can not give him yourself.
Like Savas says, cat hisses, pulling away etc, are important diagnostic points, cats do not show pain the same way a dog does, cats tend to hide their pain and can not tell you when they are not feeling well, except for doing the things they are doing like your cat. A cat can get very dehydrated quickly when not taking in any fluid, water. Is your cat sitll drinking? You can do a pinch test of the skin on the back near the neck, and if it stays pinched, like a tent, he is dehydrated and needs medical care at the vets.
If it has been several DAYS of not feeling well, I would not wait any longer, cats can get very sick quickly and go downhill quicker when dehydrated and very pale or almost white gums is a or can be a serious sign of illness. Good luck and let us know how he is.
I will keep your cat in my prayers. I love cats and all animals, but esp. cats. I have seven of my own. Fluffypurrcat
I would suggest getting her checked out. What you're describing could be caused by a number of things. I don't suppose you could provide a bit more information? How has her stool and urine out put been, and what's the consistency? Have you checked him for injury? Felt along his body for any reactive points? (Cat hisses, pulls away, etc...)