My dog was diagnosed with kidney failure, but!?

Your vet should have scheduled you for a blood recheck. If you are skepitcal then you need to discuss these things with your vet. I would start him on the kidney failure food, and any supplements your vet has given you. You don't want to not treat him because you feel he may have been disniagnosed. Kidney failure if not treated can go into very fast decline. And question... why if he went from 50 pounds to 36 pounds were you feeding him a weight control formula?? And yes i am aware of the iams recall.. at the clinic i work for we have had dogs come in that were on that formula and a handful of them were diagnosed with kidney failure. So in general you need to follow up with your vet on a blood recheck to see if the levels have improved and if not what needs to be done next.