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Could you post a few pictures of the leg, or upload a video of him trying to walk to a video platform like Youtube or Vimeo, and copy/paste the link here?
In comparison, does the leg feel warmer than the other?
Was this sudden or did it slowly appear over a few days?
Does the leg appear to function normally (bend at all joints)?
You can start off by setting him in a soft padded area, which reduces his movement, thus reducing the risk of him reinjuring the leg. A few soaks in some Epsom saltwater may prove beneficial, in addition, some administration of some B vitamins for leg health . If you don't believe there was any serious internal damage done, you can give him some aspirin in his water dish, the dose is one 324mg tablet in one gallon of drinking water.
You can do all the above, but what he'll really need is just rest.