
I would be going back to the vet for a more certain diagnosis, becasue a mobility problem (such as artritis) is different to an injury. An injury often requires rest to heal, but a mobility problem can worsen with lack of exercise.
I have an old dog (14 years) with mobility problems (he's weak & stiff/arthritic in back end) and a 9 month old puppy that has elbow displasia. From personal experience I know that exercise is VITAL, but with any mobility problems you have to be very carefull not to allow the dog to over do it, and if that means walking them on-lead at a snails pace then that is what you need to do, the pace & distance can be built up slowly as the dog shows improvements/increased tollerance. With a little bit of trial & error you can get the balance right. I've personally found that once a dog has mobility problems it is best to make sure they walk EVERY day unless they appear to have over done it then I would give 1 days rest and the next walk would be adjusted.
I wouldn't be cutting out walks completely, as in the long run it will decrease the dogs mobility further. I would keep the dog on lead until you see some improvements again and then only allow it off lead in 1 short session per walk (rather than for the entire walk), building up the off-lead time slowly.
I would also be stopping the dog from going up/down stairs and on/off furniture for a dog that is having problems walking these things can cause injury (they can slip/fall).
The Metacam can take up to 4 days to show any effects (according to the leaflet - I have a pup on metacam).
But this is all things I found best with my dogs and on my vets advice. I would recomend that you take your dog back to the vets they will be able to offer exercise advice that is tailored to your dog.
Glucosamine suppliments are fantastic :)