I'm sorry to hear about your boy, is the vet doing any further tests to try and get to the bottom of the problem?

It's difficult when they're older and won't eat, I used to boil up chicken and rice together for my oldies, so that it flavoured everything, one of their favourites.

Is there something other than cortaflex you can put him on to help his back end? I give glucosamine/chondroitin to one of my girls to help her joints. The only reason I ask is that Cushings is linked to exposure to cortisol.