16 yr. old cat constantly throwing up

My cat is 16 yrs. old and for the past several months constantly
throws up. The vet did some blood work and his sugar was a little
high, which she attributed to the stress of the visit. His kidney
function is slowing down a bit and now she wants to take a urine
sample. She has changed his food to a type that can only be bought at
the office and is a gastroenteric formula. The food is not cheap, $34
for a case of canned and $28 for a 6# bag of dry food. I don't know
what to do for him; there are times that he doesn't seem to know where
he is or who I am. Is some of this just part of the "aging process"?
He is an indoor cat, up to date on his shots and has never been sick.
I really don't know what more to do for him. Has anyone else faced
this problem or had a similar situation w/their cat?