Glad to hear that everything looks fine! No worries about not being "sensible". With the stress you've been through it can be easy to go to worse case scenario first.
When I had an old fellow who quit eating due to age, I found that mixing tuna or sardines in with his food would help. Warming it up in the microwave a bit would also help. Heat helps release the scent, which makes it more attractive.
Do they make a wet renal food? Some canned medicated food stinks to high heaven, which most cats seem to love. Even if it's too expensive to feed plain, you could mix it in. Or, really, any food he loves you could mix in. If you wet down the dry first and smoosh it with favorites of his you can probably get both into him.
And going with what victoriavictrix said: many vets say that high protein is bad for kidneys and will advocate keeping felines on dry food that is grain based, but keep in mind that a cat's natural diet is almost solely meat. Many people have had lots of luck changing a cats to their true diets. For one cat it should be less money than the specialty cat food.