I can't believe he is 22!!! That's got to be close to a record! Well done you guys for being such awesome cat parents
Anyway my understanding is that the excessive drinking and urinating is the most clear sign it is kidneys, but a blood test will tell you for sure. My cat Tuppence (13yrs - which is actually also old but sounds like she's still a kitten compared to Choc!) has just been diagnosed with failing kidneys this week. She has had the desperate water drinking and peeing up to 6-7 times a day (which is very difficult given that she is already dodgy with where she will pee) as well as lethargy, a bladder infection earlier in the year, and weight loss. The vet is very confident that the special renal diet will make a big difference and I've heard from other people who have had cats/dogs with kidney problems that the diet is really helpful. Tuppy even likes the food which is amazing! So I'm hopeful it will settle down and she'll have a few more years. Good luck!