I talked to the vet this morning and it seems that his innards are doing surprisingly well, which is what I desperately wanted to hear. She said it looks like we can safely rule out diabetes, Addison's and Cushing's. The one thing she is still concerned about is his recently developed heart murmur. She said she wants to see him again in a month, and if the heart murmur is still there (or louder) she will order an ECG.
I missed the first call from the vet, so I spent the entire journey to work having freaking fits because all she said in the voice mail was "give me a call" and I was expecting the worst. I locked myself away in a side office to make the call in case the news was bad. But, against all odds, it wasn't, only my body is still in freakout mode so I'm trying to talk myself down.