Hello, my name is***** and I have over 20 year of experience as a veterinarian. I am sorry to hear that your fellow has a red eye that has been changing over the past 4 months. Cats don't really get macular degeneration.
While trauma and secondary damage to the interior of the eye would certainly cause what you are seeing if he doesn't go outside that is less possible, but there are other possible causes.
Things that can cause eye color to change and a "red" eye include diseases that affect the eye itself, especially the retina (the back of the eye that collects light stimuli and sends the information to the brain), the nerves that take information to the brain or the brain itself, disease processes that increase blood pressure and can lead to bleeding, or increased pressure in the eye (glaucoma), or a detached retina.
In older cats hypertension, which can lead to retinal bleeding or detachment, is the most common cause of what you are seeing. Hypertension in cats is usually due to hyperthyroidism (a tumor of the thyroid gland that secretes excess thyroid hormones) or kidney failure, rarely it can be related to high levels of a hormone called aldosterone. Cats with hyperthyroidism are often easily stressed, and may have secondary heart disease (thus his heart murmur) which can decompensate and lead to the "attacks" that you have witnessed previously.
Other possible causes of what you are seeing include a systemic infection like Toxoplasmosis, feline leukemia, feline immunodeficiency virus or feline infectious peritonitis, a toxin ingestion, or a brain tumor or stroke (fairly rare in cats) or glaucoma (increased eye pressure).
Because only one eye is affected a tumor is also possible. The most common types of tumors in the eye include melanoma, lymphoma or a squamous cell sarcoma.
With some of the disease processes I mentioned, if controlled, changes to the eye are at least partially reversible and he could regain some vision but we need to act quickly in order for this to happen. It may be too late to regain what he has lost now in the original eye, but perhaps we can save what vision he has left in that eye or save the other eye.
He should see a veterinarian as soon as possible to have some testing done and start treatment immediately. I realize that won't be easy for him so perhaps a mobile veterinarian that can come to him would be better.
Please let me know if you have any further questions.