Update: So, I decided not to go into the vet early after his urine seemed normal again (though less quantity as I think he has been drinking less). Then I somewhat regretted that decision as it seemed like his energy levels declined greatly on Tuesday and I wasn't seeing him at all- but Monday he seemed to be coming out frequently, so I had made my Tuesday morning decision based on Monday.
In any case, I do think it worked out alright. I was pretty sure he was on death's door...when I would open his hideaway ceiling to check on him he was super slow to react, but then later Tuesday night my daughter called me in to say that he'd come out! Physically, he did seem worse for wear- definitely seeming to flop to the right more frequently and generally have less control. But mentally he seemed like himself- looking to us for food, eating out of my hand 2 times...
So this morning I took him to the vet and again, I could see that physically he was really struggling- circling to the right more out of lack of control than anything else, but again, mentally, he was behaving like himself- we had to drive 35 minutes and for most of the time he was trying to peek out of his carrier- he almost never settles down in in.
At the vet I declined an x-ray (as even though it would have given them more detail, the doctor didn't think it would change treatment), though I can opt for one in 2 weeks if I want when I return for a check-up. She did not detect signs of ear infection in the inner ear, but she did prescribe Meloxicam, Gabapentin (sort of like Meloxicam but more targeted for nerves- I have to wait to get this shipped), Enroflaxin (an antibiotic in case an imperceptible infection is a source) and Benebac (probiotic).
I'm feeling hopeful that we will see improvement in his physicality, even if he is now living with some long-term changes.
He is currently sleeping in his carrier and I am keeping an eye on him- I don't have the heart to move him back to his cage immediately. Even if he just goes to sleep, I feel bad rustling him when he's had such an eventful morning and I'm sure he's getting much needed R&R now. They did administer his first medicine doses there, with the exception of the Gabapentin. This was my first trip to this animal hospital, which is the closest one specializing in "exotic" animals. It was good to get there though and be able to have a feel for a place that specializes in rodents, rabbits, etc., and came recommended by our local shelter that specializes in rabbits.