Take him in for a full blood panel & a urinalysis......honestly this should have been done before the vet did the xrays.
You need to have the vet keep a close eye on the hemocrit/PCV - Packed Cell Volume on the blood test - this will determine the amount of red blood cells your cat is producing, if not enough the cat is suffering from anemia.
Anemia will show signs of weakness in the legs esp the back legs as well as the spine, pale gums, litter eating & concrete licking among other symptoms
Another cause of litter eating is mineral esp iron or calcium deficiencies: http://www.pets.ca/forum/showpost.ph...3&postcount=16
A fecal test should also be done to determine if there are any parasites such as tapeworm that could be causing anemia