my cat

That's a lot of weight loss. Lethargy means no energy, doesn't want to move, just lies around. Lethargy and inappetance definitely sound like something's wrong. What kind of tests did the vets run? I would want his kidney function checked at a minimum (blood tests and urinalysis). How long has he been off his food? How long did the weight loss take? Are there any other issues? Is he peeing and pooping normally?

He has detached retinas in both eyes?

I agree that it could be kidney disease.  Sudden weight loss, no interest in food, Even the blindness.  (kidney disease is often accompanied by high blood pressure, which can lead to blindness, which BTW,, CAN sometimes come back if the high BP can get back under control)  I am stymied too by the fact that he has detached retinas in both eyes!  That seems very odd.

It's great that he isn't bumping into things!  Maybe he's only blind in one eye?  When you shine a light in his eyes, what happens.  Do either of his pupils change?  Since he's been to two different Vets, did they run any bloodwork.  What did they say other than "he's blind because of detached retina".  Did they mention hyperthyroidism, or diabetes, or kidney disease or anything else?