Good news to report: had our follow-up visit with the vet this afternoon and the vet says he is definitely doing much better. She didn't even need to take x-rays, she said she could tell by feeling his belly that he was improving; it was much smaller and there was more poop in his colon. She also liked the quality of the poop he was putting out. She said to finish his course of meds and come back next week for a follow-up unless anything changes for the worse.
Needless to say, I'm relieved. I knew we were pretty much out of the woods considering how much food he's been inhaling, but it's good to know that everything's moving inside him and not sitting there and becoming obstructed.
Now comes the hard part, though; getting His Majesty to accept that he's not going to be getting so many treats anymore. Somebunny is going to be SERIOUSLY grumpy when that sinks in.