I know that we will all be praying for God to ease your brother's pain. Yes, he does have a good heart. It is sad that a transplant will not work at this time, but I am sure that God has a plan "to give a future an a hope" to your brother, whereever that future is. Also, if your brother thinks that he may not live much longer, it still might be worthwhile for him to think about exploring alternative medicines or trying a change of scenery. While this may not prolong his life, such measures might be very helpful in relieving pain. Is there anything that he's really wanted to do for a long time? He should go and do it, if possible: this can really help with quality of life and closure. I would encourage you to encourage him to feel free to do this for himself.
One thing that I'm glad to hear for you is that it sounds like you and your brother have a very good relationship. That is a treasure: if your brother's time to go to Heaven comes, at least you both can say goodbye to each other in peace, knowing that you expressed love to one another.
Whatever your brother decides, it seems like he's really thought about what he wants. I will say that if he should decide that he would like to explore transplant options, he could always go to another doctor for a second opinion, as prognosis in the cases of people with spinal cord injuries are often bleak, even from the best doctors. Sometimes other consultation indicates a much more hopeful outcome. What I mean is, his decision not to do a transplant is perfectly okay if that is what he wants, but I hope he doesn't feel like it would be wrong for him to recieve a transplant: he's not obligated to give his life because of his injury, his life is just as important, and he certainly has a right to explore all options. But, it seems like your brother has been trying to make some very serious, difficult decisions and has been trying to do so altruistically. Has your brother looked into home hospice care to help with pain?
I'm sure many of us will be praying tonight before he goes to do the doctor tomorrow. And, once again, please let us know if we can do anything to help your brother in terms of providing advice, support, or assistance in anything.
I know that God has a close eye on your brother and know that he will be right there with him, esp. in the persons who love him, like you.