These are the reasons that I can think of for rear leg/tail paralysis in lizards: tumor, fracture, Vitamin B1 deficiency, calcium deficiency (MBD). Given that tumors are rare and there is no history of a fracture, I'd lean toward dietary issues.
Is he a rescue? How long have you had him? Is his basking area 95 degrees and is the lighting spaced appropriately so that his UV absorption is high enough (ie not through screen/mesh and not too far away)? Does he get supplementation and a variety of foods that are not high in oxalates?
As for treatment, I'd get him in as much natural sunlight as possible. I'd also get him to a vet, because if paralysis is setting in, then anything going on is pretty significant. In other words, the amount of vitamins and minerals required would be larger than could be absorbed from the gut. If you think he's constipated, try a warm bath. It would take a large amount of fecal matter to apply that much pressure on the spinal cord and outlying nerves.