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If this is a sudden onset shaking then there could be a number of causes, including muscular weakness causing tremors, and pain or inflammation within the muscles. If he seems otherwise well in himself, except for the legs shaking then its likely to be a neuromuscular problem (either connected to the nerves innervating the muscles of the legs, or the muscles themselves). However if he seems down in himself then it could just be secondary to some other underlying problem.
If his legs continue to be shaky, then it would be worth getting your vet to check him over thoroughly, as there are so many different possible underlying causes to this that its hard to speculate- however, if it seems to be purely affecting his hindlimbs, then it could be also related to arthritis within the spine (spondylosis) which is common in his breed and can cause these symptoms- in which case buffered aspirin at a dose of 5mg/lb every 12hrs with food may be beneficial.
If you have further questions or information on his status please let me know!