My 14 year old cat was diagnosed with the same problem in September. I know it's a horrible, frightening thing to hear, but don't become despondent; there's quite a lot you can do to help, if it's caught in the earlier stages.
My cat is on Royal Canin Renal diet. She tried the moist version first and hated it, but fortunately likes the dried one. It's ok to give dried food, if that's what the cat prefers, as long as they are drinking a large amount of water. I think most cats with kidney problems will want to drink plenty anyway, as they will be feeling more thirsty. All the information I have read says that whatever food they have needs to be lower in protein, and very low in phosphorus.
I think it's best to be a little wary about what the vet prescribes. I'm not suggesting that we don't need their help and advice, because obviously we do, but unfortunately vets, like doctors, will often prescribe drugs without worrying about the side effects. My cat was given Nelio tablets, and when I looked online, I discovered this medication can actually cause kidney failure in higher doses. Also if the kidneys are already not working as efficiently as they should, medications add to the number of toxins they have to try and eliminate. I would't rule out traditional medication, if it was the only thing that would help my cat, but I would always take the natural option if there is one available.
My cat is taking two natural products for her kidney problems which I can really recommend: Denes Kidney tablets (from
www.denes.co.uk) and a liquid supplement called Kidney Support (from
www.petalive.com). Both of these contain herbs which help the kidneys to work more efficiently. Sorry, I know you asked about natural products for arthritis, not for the kidney problems, but I just had to recommend these two things, because they seem to be helping my cat so much. She is acting normal, still wants to play with her toys and run around (as much as you would expect for an older cat), and has a good appetite. I'm not suggesting they are some kind of miracle cure of course, but I think they can help a great deal, just going by my own experience.
Before I found these two products, I also tried Astro's Oil (from
www.astroscrfoil.com), but this did not help my cat. She became more lethargic and kept scratching herself. I suspect she may have been allergic to one of the ingredients, because I have heard of it helping other cats.
The other thing I would say, is try to avoid your cat becoming stressed by anything if possible, such as other cats visiting, etc, because everything works less efficiently under stress.
I can't recommend something natural for the arthritus based on personal experience, but in a book called Natural Cat Care by Bruce Fogle, it suggests Acupuncture/Acupressure, and essential fatty acids. I know that you can also get Glucosamine supplements for pets, and I expect both Denes and Pet Alive might have some kind of supplement for arthritis. I have read that the following herbs can help cats with arthritis: meadowsweet, cleavers, comfrey, cowslip and rue. Apparently you can also use these herbs in a poultice, although I do wonder how most cats would react to that.
Sorry to hear about your cat's problems. I really hope you find something that works for him.