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Knuckling over of the feet or toes can indicate nerve damage, either in the affected leg, or in the spinal cord.
Also, older Goldens are prone to arthritis, which can lead to weakening of the muscles and knuckling due to that weakness. Without actually examining him myself, it's difficult to say what is going on. There are many therapies out there that can help arthritis or nerve damage, depending on the diagnosis. You should have your vet look him over to make sure he is not in any pain.