Yes, he has been to the vet, lots of times. I have a post about it in Cat Health.
http://petoftheday.com/talk/showthread.php?t=120176

Our last visit, which was Feb 13th, she said to feed him whatever he wants, just to get him to gain weight. His weight was down to 12.7 at that visit. And even she said that was too much weight lost, too quickly.
She gave him a cortisone shot on Jan 30th to help increase his appetite. I think that has just about worn off by now. So his increased appetite is a good thing now. Yeah, I don't want to get back to the 21 pounds like he was before, because that wasn't good either.
But anyway, he has had blood work done many times. And everything came back fine.
OK, one more question. If a cat has diabetes would that increase or decrease their appetite? I have noticed since he is finally eating, wet food, he seems to urinate more. And I know that's a sign of diabetes, but they checked for that when I took him in on Jan 23rd.

Oh, to answer some of your questions...he is almost 12 years old and has always been a little on the heavier side.
And the dry food - he is not eating a lot of that. I only put 1/3 cup in the dish and only refill it when he has eaten most of it, which has been probably every 2 to 3 days lately. So he's not eating too much dry food.

Please let me know what you think, and if you have any more questions.
Thank you for your responses.