Have you tried putting a "low" heating pad or warm Snuggle Safe unit in his favorite spot, plus a soft, fluffy bed? Then give him all the rubs, scratches, and love you can. Extra-yummy food might help, too.
While you might not fix anything with it, I bet it'll fade in his mind; I've seldom met a kitty that didn't feel better with spoiling!
A human amputee posted about phantom pain relatively recently, and CarolC found out that the newer family of drugs used in humans with chronic pain, such as Neurontin, can be used in critters. I'm NOT SURE if that means kitties, since many things puppers take kitties can't, but it's worth looking into.