Please take a look at this site: https://edis.ifas.ufl.edu/ps031
We can rule out ammonia toxicity, avian pox, developmental disorders, and avian encephalomyelitis out of hand. A nutritional deficiency is unlikely if he's made it to 7-9 years of age. This leaves age-related changes such as cataracts in an elderly rooster, bacterial and fungal infections that have not only involved the right side of his face but also his eyes, neoplasia (cancer) that has not only involved the right side of his face but also his eyes, and the elephant in the room - Marek's disease. In fact, blindness should be assumed to be caused by Marek's unless proven otherwise. The kicker here is that Marek's - while it can affect any age chicken - is far more commonly seen in immature birds. Nevertheless, I've seen variants of this herpesvirus in older birds.
Pragmatically speaking, Biscuit isn't likely to be diagnosed accurately without the aid of an avian vet (please see here: www.aav.org) who is also adept at using an ophthalmoscope to examine a chicken's eyes. I admit that that's not commonly done but should be. In fact, the etiology of his blindness might not be determined pre-mortem. There's no treatment for Marek's. If the swelling indicates neoplasia Biscuit gets to be spoiled inside the house until his quality of life isn't acceptable. His weight loss might indicate neoplasia but his inability to find food might well be the culprit too.
Please take a look at the eyes of many Marek's patients here and tell me if you see changes such as these: http://www.birdhealth.com.au/#!mareks-disease/c1qzk I'm going to run Biscuit by a friend of mine who's a specialist veterinary pathologist with an interest in birds.
My friend said the following: "He's an old guy and some old age disease, like cataracts (although they say it was sudden onset) or neoplasia (such as leukosis or Marek's with the weight loss) is my suspicion."
Please respond with further questions or concerns if you wish.