Hi There Cheryl,
Sorry, I've not been reading the group regularly, another member (Rebecca) alerted me to your post to see if I could help.
I'm sorry about Jack's problem; I did go through the very same thing with my Cocoa, though not until a bit later in his life. Five does seem rather young to me, but rabbit's style have somewhat mysterious physiologies. *sigh*
Anyway, my first advice is for you to join the disabled rabbit's group on yahoo.
http://pets.groups.yahoo.com/group/disabledrabbits/
I'm not trying to scare you or anything, but all the questions you're asking here, and this is a great group, but most of the people there have already gone through it with their buns and they have lots of experience with these issues.
Someone else posted about spondylosis and bone spurs in the spine, those are likely causes. Sometimes bunnies injure themselves and that starts these conditions in motion; sometimes it's just genetics.
There are other meds you can try with your bun that have helped others: Adequan comes to mind. I don't have personal experience of it myself. Acupuncture has been helpful to many other bunnies.
I found with Cocoa that encouraging regular, gentle exercise helped keep him mobile for as long as possible. you want to try to keep his muscles strong for as long as possible. The muscles usually grow weak, because the nerves to them are being pinched off in the spine. It usually is a progressive problem, so you need to be ready in your mind to help him stay mobile for as long as possible.
I added ramps in the house, cut the side of his litter box, eventually he had a ramp to his box.
The most radical solution people have done on younger bunnies is that eventually the leg gets in the way of them walking, so they have been amputated. Now this sounds harsh, but if the issue is only affecting one leg and the disease doesn't progress further, it helps keep the bun mobile for much longer.
Those are just a few thoughts...please join up at the dr group and explain where you're at now. They will be very supportive.
When my Cocoa was at Jack's stage or a bit worse, I made a panty hose sling to go around his thigh to hold his leg up so he could walk around freely. It worked really well unless he wanted to go under things...lol He loved his outdoor walks.
The metacam will help to slow the progression of the disease.
Ok, bye for now dear...take care.
God Bless, Arlette