It's not feline diabetes! He has hyperthyroidism, which lots of cats live with without any intervention. Being as he's considered "elderly", and in very good health otherwise, the vet was OK with just taking a "wait and see" approach. Because I'm "all" about the "quotes".
The weight loss should stabilize, and as long as he's playing and behaving normally, we're good to go for a while yet. He's 14, and could have a couple of good years left, so we're feeling pretty happy.
(and, I'm leaving for two weeks on Friday. Have I packed anything? Nope. I'm blogging instead. I'll never learn, I guess.)