I did a google and read a little, too. I also found thyroid could be involved; hyperthyroid was specifically mentioned; is yours hyperthyroid or hypothyroid? Evidently, hyperthyroid can be associated with detached retina in cats. It is possible it will result in blindness in that eye, so you might want to be prepared for this. Although, in your situation, it does not sound like it would alter his quality of life much.
At any rate, the bottom line is, my google search indicated there was no treatment available, and I found nothing to indicate pain might be present; probably not different from what your mother found. So, no emergency, and no pain.
I am a retired nurse. I never saw anyone with blood in the eye who found it uncomfortable, and I have seen quite a number of cases. Hopefully, cats are no more uncomfortable with it than people are.