impaction can also be caused by feeding crickets that are too large, not enough water, either via misting or having available drinking water, or ingesting substrate.
for minor cases of impaction, warm soaks in water at about 70-75F for about 10-20 minutes can aid in passing of stool, mineral oil as heartmountain already mentioned, or pureed carrots or squash can also help things flow. a teaspoon of puree a meal for a couple days is more than enough. if none of this works, y our only option is taking the dragon to a qualified herp vet experienced in working with dragons and other mid sized lizards.
as he is already showing signs of rear leg paralysis, i am more inclined to suggest getting him to a vet right away, especialy if he doesn't go to the bathroom today after using above mentioned methods. too large prey items certainly can put pressure on the lower spine in young dragons, cau sing the back legs not being able to work properly and that little hump. also, lack of calcium or vitamine d3 can also cause problems like this.
in fact, as this being friday and the fact many vets are not open on weekends, i suggest getting him in today anyway, as in the worse cases, a wait till monday may be too late to referse damage to the spine, especially if t his has already been happening for several days before you posted here for help. if you need help in finding a good herp vet, try this link.
www.herpvetconnection.com/
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PHLdyPayne