Definately take him to the vet if you have not already done so! Not to scare you or anything but I had a cat about a year and a half ago that started being more vocal, and it turned out she had kidney cancer. She was being noisy because she was in pain. Oddly though it did not sound like a 'painful meow' at first. If everything checks out okay, then I would get her a kitty companion. I did with my other cat, and it was the best thing I ever did (after a little adjustment period of course). Good luck, and let us know how it turns out.