Care and medical needs for senior cats

We believe my once-"baby" stray cat is now about 17 years old. He's had digestive problems for several years but no cause found. Treatment with usual hair ball remedies.

Then in 2008, we see he's lost weight. Sept 2008 blood tests showed he might be going into kidney failure and he had lost 2 pounds. Regarding the vomiting, Vet stated older cats often poorly digest dry food. Recommended wet food, which we have done; 1 can a day.

Cat is eating much better and rarely throws up now; he drinks a normal amount of water. But he has almost no fat now on his haunches; I can feel and see every bone down his spine. Also, he is itching severely x1 week; even just to touch his neck triggers his rear leg to start scratching. No fleas; a strictly indoor cat. It is winter and we have dry heat, but I also know itching could be a sign of kidney issues. How can I help him itch less?

I don't want him miserable or expending energy and fat reserves unnecessarily. And I'm not ready yet for "my baby" to die (from old age or illness). How can I support his health and promote weight gain in my elderly cat, while also supporting his aged & ailing kidneys?

Can you also discuss care and medical needs of senior/elderly cats, please?

Thank you.