Give plenty of fluid (ie water) to keep him well hydrated. Bathes can help with this. However, also make sure he is getting good basking temps and ambient temperatures in his cage. I suggest basking site between 100-115F with ambient around 85-90F but still have a cool end for him to return to.
If the rear leg is definitely from a too large food item, doing the above may allow him to digest the food enough for it to past. If you are not sure what is causing the impaction./rear leg paralysis...you can still try the soaking/hydrating..but if he doesn't pass it in a couple days..or show signs of stress or straining when pooping..take him to a vet.
Soaking, good basking temps etc, only work if the impaction is minor and food related. If its impaction due to ingestion of substrate or other foreign matter, then a vet trip is definitely needed, and in worse cases, surgery to remove the blockage.
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PHLdyPayne