12 yo rabbit with one numb leg - looking for some advice

Long story short after our holiday trip, just a day before we came back, our caregiver found our rabbit in a weird position, which later confirmed to be a likely spinal cord issue (no visible fracture or spine damage), which made one of his legs go numb.

Now his sister actually broke her spine 4 years ago, we gave her few more happy months taking care of her (she actually learned to move around and hop after 3 months), but obviously it’s a different thing with a 4 years older rabbit

So far we were not given any definitive answer why this could have happened, and we’re definately not putting him down until we’re sure we cannot give him some more time without huge pain or discomfort, or that this is truly irreversible.

So, my questions.

• anyone had such a case and could point to a set of possible causes, that are not spinal cord related, that we can suggest to our vet?
• did anyone take care of such rabbit? Right now he obviously does poo and pee underneath himself, but does “one legged rabbits” actually learn to move around with just one leg?

we are considering to put him down this time, as so far he is MUCH more scared than his sister, but not until we exhaust all the possible ways out of it.

Just a note: his sister broke her spine because of 2 story cage. Since the incident, we got rid of it and he’s sitting in a single story one to avoid this. My own learning I got years back: don’t give older rabbits multi story cages, as if they hop off it the wrong way they’ll end up like this.