Take him to a different Vet and don't mention the first Vet's diagnosis.

Obviously, he has had a kidney transplant. The question is: How did he get
to India, and how did he manage to pay for it? Better check your VISA
balance........

I'm very happy for you.  It happened to me several times with one cat
who would take off every summer for two months and come back when the
weather chilled out, but she would always come back looking good and I
wondered who her summer family was.  Never found out, and though I put
a collar on the cat, she never stopped going, so they must have been
lots of fun.  They didn't seem to mind her new collar either!

By the way, if you're taking him for a re-check on that diagnosis, I
wouldn't hide the fact that he was previously diagnosed with renal
failure.  You do the cat no good not telling the new vet everything
you know.  Mention the diagnosis and the subsequent events and the vet
should be reasonably intrigued to do a good job investigating it.
There are forms of renal failure that take a long time to deteriorate
to the point where the cat cannot function, and don't affect urine
output to the point where someone would notice right away.  There's
nothing inconsistent in your cat still doing well a year after
diagnosis.  And even your current vet might be interested to hear,
without anyone assuming that he's incompetent or wrong.

There's also nothing wrong with a second opinion, but if the diagnosis
is a tricky one, new vet deserves all information available and a copy
of the old labs.  This is not an opportunity to "test" a new vet, you
want to get to the bottom of this.  The old labs might be crucial to
compare to the new ones to decide if there has been any long-term
subtle changes in function.

Also, if the second vet turns out to be a good one that you want to
keep, you will be getting the relationship off to a rocky start if you
start by playing guessing games with him, and he subsequently
realizes, after diagnosing the cat, that you must have known all along
and were withholding information.  They WILL ask you what his medical
history is, and won't be impressed if they find out later you were
playing games.

Good luck to you and kitty

--tension