Jack has lost the use of his back legs - advice?

One of my sisters buns, Jack, a lionhead has lost the use of his back legs. Last week he started getting wobbly but this morning she found him in his bedroom with a wet bottom so he'd been there for a while. He tries to pull himself along but the weight of his legs are too much for him. The vet said it's old age but he's only 5 but has given him some anti-biotics incase it an infection.
She has moved him indoors so he can be warm and comfortable.

Does anyone have any ideas? Will he get better?

Did the vet suggest a Panacur course?

Sounds like it might be EC. Has the vet said to give him a course of Panacur?

Also, whatever the cause, I'd say anti-inflammatory/pain relief is a good idea.

Has a spinal injury/disc prolapse been ruled out ? Did the Vet assess his neurological responses ?
Has he had any anti-inflammatories-probably a one off steroid shot in the first instance. A NSAID for more longterm use.
Covering for possible EC would be a good idea, so a course of Fenbendazole (Panacur/Lapizole)

If he has lost bladder control he may need it expressing manually, has the Vet addressed this issue ?

This happened to my bun too, I took her to the vet with her wobbly self and he told me it could be either e. cuninculi (a parasite) or a prolapsed disk in her back. He put her on anti-inflammatories for the disk, and antibiotics and panacur for the e. cuninculi. One of them did the trick and she cleared up after a while.
I'm afraid it sounds as if you vet isn't very rabbit-savvy, lots of vets are more cat and dog oriented. Ask if there's a vet with a special interest in rabbits or you could take along information found here and mention it carefully, without sounding like you're trying to upstage the vet.
No-one here can guarantee he will get better, he could really do with seeing a specialist rabbit vet, plus some of the medication I've mentioned is prescription only.
In the meantime you can do what you can to make him comfortable, bringing him inside is a great start. Make sure he isn't sitting in his own pee and keep that area as clean as possible, I bathed my rabbit and used a tea tree cream (babies sudocrem does the job) to stop acid burn from hurting her legs. Nappies also work well if you can get them to stay on, Vinnie became a master at pulling them off unnfortunately! I also used incontinence pads for puppies on the floor of her cage so at least she wasn't sitting in her own mess.
Sorry for the essay but I've had a few bunnies with similar problems and it took a while to find a decent vet. Hope some of this helps.