Oh dear , from what I just read your cousin's chances are very very poor. That is a really aggressive form of cancer.
It makes sence that it spread quickly to his lungs and from the nature of the cancer I'm not overly surprised at the amount of tumours found. The cancer forms in oxygen rich muscle cells, so it will show up anywhere.
I really wish your family well and that they get through this. But their descision to not tell your cousin that he is terminal is very very wrong. He will be disaapointed to learn that, but the joy and strength your family will get from seeing him fight and strive to squeeze everything there is from life will FAR surpass the need to protect him.
Besides I am sure that he knows what is going on anyway.