He has paralysis of the left side of his face, triggered by a very stubborn middle ear infection. This means he isn't able to close his left eye, making it vulnerable to ulceration, etc.
Oh dear. It's all rather worrying. My head tells me that it may have been kinder to have him put to sleep, but my heart tells me that he's not ready to give up yet, and neither am I. Fingers X'ed. Pets, eh? It's just as well we love them.
And to change the subject, this made me laugh: