S.W.
You have a hard decision in telling him about his life. He may or may not react very well to the news. Be prepared for any anger or shouting or withdrawing from society. It is part of living with an illness. The best thing for you is to be there and not be judgemental. If you can get the other members of your family together in a family meeting call one and explain what has happened and how you are all going to handle the remaining days.
The doctor is right about your brother making the decisions on his treatment. We as family members want the member to do everything to stay here with us but we do not know when we will be called home and that is the hard part.
As a couple of the posters mentioned, make a video or a photo album up that he can look through and reflect on fond memories. Also do get a list of all financial and medical especially the HIPPA documents and wills and things including computer passwords and put them all in one spot so that when things are needed you will find them.
Above all, live each day as you have lived it to maintain a sense of normalcy for him. Don't dwell on the upcoming but on the present. Take him out to smell the roses and see the sights and smell the air and enjoy a warm day as they have new meaning to him now. In fact do a picnic with the blanket and basket under a tree in the park. It is one that you can not understand unless you have been down that road and survived.
I too am going through something similar and am looking at each day as another day of having hubby in my life and at home.
Just give him a sense of peace and dignity.
I will keep you in my thoughts. The other S.