I'm so sorry that your brother is having to fight this fight.
What I would say given the information that you've given us is do as Sue said and make sure he knows that there are options available, and help him to sort through those options. Let him know that you stand by him no matter what. Be a person of hope when you are with him. Believe that he can beat this, and let him know that you believe that.
That doesn't mean that you shouldn't also be a sounding board when he needs to talk about how defeated, or weary, or tired, or angry, or depressed he feels. All of those feelings are going to come up and they are normal, and you want to be careful that you let him know that you recognize the validity of all of those... but still have hope.
Keep coming back here. There are lots of good people, with amazing insight and encouragement.
Hang in there, and know that we're here.