Lets break it down.
I've got a life threatening disease
OK, This is not a pleasant place for anyone to be. There is nothing that anyone is going to do to change that though. That fact is not a tool that should be used to hold people emotional hostage, which is sounds like he is doing. You have to be aware of that and protect yourself against it.
The doctor told me I should talk to Mom about it, but Mom won't talk to me, says it not her problem.
Quack. If the doctor said that specifically he isn't giving the doctor the complete picture of his reality. Your mom is holding her boundaries - good. It always gets twisted around to this topic doesn't it? Put all this into the 100% ignore file.
My choices are bad and very bad. Take the treatment that will destroy my immune system and I start contracting anything that I get near, or destroy my ability to breath and die.
Treatment decisions are between him and his doctor. He is filled with so much self pity and manipulation it is hard for me, stranger on the internet, to muster up any compassion and empathy for him and I'm not the one being manipulated. I can see why this is a difficult situation for you. I think a simple "I know these decisions are hard" or something like that will show you empathize without getting into his web of self pity and manipulation.
You know the doctors visit is not going well when the doctors says I should get a living will.
Ignore this. As mentioned everyone is told that, especially an older man with a chronic health condition. Doesn't mean he'll die tomorrow.
His acceptance of his health condition is his work to be done. If he would rather use it as a weapon in his manipulation arsenal then he'll do that but you can detach from that.
My life really sucks now!! I can hardly see, I can't lift my arms, chew my food, and have a hard time breathing and Mom won't talk to me.
'I'm sorry about that. ' THE END. and again - always comes back to mom won't talk to me. Ugh. Is the stuff before that even true?
I'm not going to able to get my own insurance to pay for treatment,so I guess I'll just f*ing DIE!!
:drama:
Quack. Since you've done the research you can send that to him but I would not expect anything other than more of this. He is playing it.
Maybe Mom wants me to die so she can collect my $300,000 life insurance policy. Maybe I should make you the benefactor.
Drama. He can do whatever he wants with his life insurance. I wouldn't touch this with a 10' pole.
Hang in there. Break it all down. Respond with empathy when called for and completely ignore all the rest.