My mom had lost the sight in her left eye, permanently, due to stroke, high blood pressure. She now has poor vision (almost legally blind) due to her HBP, and diabetes.
Not that her loss is the same as your brother-in-law's, but I noticed since that happened years ago, she too, started to withdraw, eat more and just basically give up on life. . . depression she was in denial over, I had suggested she try to join some support group, being with those that have or are going through a similar situation may have helped her to find ways of working through this, but she was bitter, which is understandable, it is devastating to lose one of our senses or body parts. It may help if your brother-in-law gets some sort of counseling, help him not to isolate himself, perhaps if this is started sooner than later, he will not continue to fall victim of dark depression, we all need help at times, sometimes a professional nudge.
It sounds like you are loving family, and you adore him as you said, has anyone spoke with any of his doc's?
He can chance losing his good eye to HBP and the many complications of HBP and obesity, that would be tragedy upon tragedy. OKay, I am going on too much here, but if I werein your place I would sit with him and cnvince him he can get some help adjusting to this, it is hard to accept but now we can't go back and change what had happened, but try to make him understand that he still has a life living, and he can go on well in time, especially if he takes the weight off.
Maybe someone in the family can be a walking buddy and make a time to go together for a nice walk?
Just throwing some ideas out there.
I wish him luck, it has to be so tragic, I know my mom grew more and more frustrated only having sight in one eye, cause her depth perception got all off kilter, she slowly got around, I still feel that had she got into counseling and support groups, maybe one just for visually impaired it could of helped.
See if there are such things available to him, and have the family share some of the running around, that helps too.

Please take care,
DE

In giving advice seek to help, not please your friend
SOLON