I’d give him a chance with cage rest, if he seems comfortable. By a dose for 8 kilos I’m guessing you mean an 8 kg dog? (Or is he a very big rabbit?) Rabbits metabolize metacam a lot quicker than dogs and cats, so they need a higher doser per kilo body weight.
Rabbits have good bone-healing capacity, unless the break occured due to bone malignancy (which is very rare).
Giving him cage rest is an absolute necessity for the nearest time. He must put as little weight as possible on the broken bone. The bone endings must be aligned for the break to heal, so he cant be allowed to hop around. You have to help him with his hygiene, and and you could add a probiotic (f ex benebac) to his food or water to help his tummy.
Bone is made up of mineralized connective tissue (which in turn is made up of proteins). If his tummy can deal with it, you can give him food that is slightly higher in calcium and protein. You dont want him to get fat though, so you must be calorie-cautious. Some alfalfa hay (not a lot) could be a good source of a extra protein and calcium. It also has good fiber.
Here is a link to Disabled Rabbits, which is a website for owners of temporarily or permanently disabled rabbits. It has good tips, for example on housing and hygiene.
http://www.disabledrabbits.com/