My son is very much like your brother, but somewhat less degree. He also will shut me out instead of listening to me. I can easily imagine how he would think his life is over if his brain is damaged, so he is guarding his deficits. He does need to know that it is counterproductive to do so, and that with cooperation and accepting help and therapies, he WILL get better, it just takes time to heal. It sounds like he will not take this from you, so don't try, but he should be seeing a good psychotherapist to get him to accept and move on and convince him that life will be good again.
I can't ever imagine "letting go" of my son, it seems to me it would be a death sentence. You just know he will sooner or later make some major error that could be fatal -- like falling off a ladder that nobody in their right mind would be on in his condition. (oxymoronic phrase?!) People have the right to make their own decisions, but they also have the right to be protected from themselves when they are in such a state as to cause themselves much harm. So just like the TBI mantra goes, "NEVER GIVE UP!" I'm not giving up. You're not going to give up, find someone professional to convince your brother to Never Give UP. Best wishes!!!!!