Amputation seems like a major step for treating arthritis, especially in an 8-year-old rabbit who will be under increased risk when going under anesthesia. I'm not sure a vet ought to make such a drastic claim before even seeing an X-ray to confirm what the problem with the leg itself is. Amputation is a very viable option for many leg injuries (and certainly could be for your rabbit as well) and quadrupeds tend to do very well missing one leg, providing that the other three are stable and not too arthritic and the rabbit is kept at a leaner, healthy weight to prevent too much strain on the remaining joints. If this is the route you choose to go it could improve his quality of life. Have you tried any analgesics (painkillers) or steroids (not many are rabbit safe and those that are may not be helpful for arthritis, although it's still worth asking) to reduce the symptoms?
I am not a veterinarian, and no advice I give should ever be taken as similar to the weight you should give your vet's advice, however, I would seek a second opinion before undergoing such a drastic, risky procedure to see if there are any safer, less invasive avenues that may yield equally positive results.
