I have trained several cats to walk on a leash in a harness. The trick is to start them young and get a cat harness. They wiggle out of the ones meant for dogs. They wiggle out of the ones meant for cats too just not quite as fast.
Next cat I am thinking of getting a "Kitty Holster". George T. Cat is now starting his 15th year of life and I don't think it would be fair to start trying to change things at this point.
I wish I’d harness-trained mine - my Tomcat would love to meet more people. Ah well, so it goes.
Best of luck to you and George.