Hello David, I'm Dr. Deb.
I recently came online and see that your question about Cooper hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
The most common cause of increased appetite in an older cat is going to be Hyperthyroid Disease. I've seen a few cats with Diabetes have increased hunger as well although most of them will also have increased thirst/urination. Not all of them do, though, so sometimes it can be difficult to tell if they have an elevated blood sugar level without running bloodwork.
Cats with Hyperthyroid disease (LINK) will often have other signs as well such as weight loss or vomiting but I've seen a few who didn't present in a classic, text-book way.
Routine blood work usually includes a T4 which is often elevated or at the higher end of normal when this condition is present. Occasionally, additional testing is required if the T4 is inconclusive.
Unfortunately, there won't be any other easy way to tell if Cooper has either Hyperthyroid Disease or Diabetes without running blood tests. We do have treatment options for both of these conditions, though, which can improve the quality and quantity of an older cats life once initiated.
I hope this helps although, again, my apologies for the delayed reply. Deb