Is my cat going to be okay?

My cat has two "third-eyelids" and one is covering over half his left eye, the other is about 1/4 of his right eye. He went to the vet for a past illness, and he is now getting eye drops also for this in addition to other medication for his sickness.. At first, when he came home, his eyes looked a lot better, but they are getting worse again. I am thirteen, and my mom says he will be okay, But I have no idea if she's just trying to be comforting. He is twelve years old and been an outdoor cat for a couple years now, but the Vet advised to keep him indoors from now on. I just want to know, if it's possible for him to pass away or be in enough pain to be put down from this these third-eyelids...I want a real answer, no matter what it is. Please, if you can answer this, do so