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I would definitely have hm checked for a back injury or a disc problem. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. You can read about this here:
A common issue in dogs with disc problems is their reluctance to jump and use stairs. Many owners have a little set of stairs at the bottom of their bed to allow a dog the ability to get on the bed without having to jump up. If the dog already has back problems, they might have a carpeted ramp.
Before you can have him seen I would be sure he has strict crate rest. Only allow him out to eliminate and only on leash. If you have to lift him, be sure to support his back legs with one hand and his chest with the other to avoid stress on the back. I can not comment on the metacam as you really shouldn't give prescription drugs to your dog without notifying your vet. Some drugs can cause organ failure and thus bloodwork is done prior to staring or restarting them on a medication.
I would have him seen as soon as you can and keep him calm and inactive as possible until then. I hope this information is helpful to you.