Thank-you to everyone that posted kind words of support and/or tidbits of advice with respect to my last post. You all realize how important an animal friend can be - there is a true bond.*
I called the vet to book Simon in for a bile acid test (to test liver function) and a Cushing's disease test. So, he goes in on Monday and hopefully it starts us down a path toward some definitive answers. I'll let you know what happens as we go along.
Again, thank you.
*Anonymous #2 (from the comments to the last post): I'm not really sure how you thought you were being helpful. For those of us with a soul, giving up so easily on a beloved family member is not an option. Sheesh.