A question regarding cancer and not knowing the right thing to say..

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I recently just read a topic about cancer, and it made me think.. Someone VERY much close to me, has an eight year old brother who means everything to him. They're very close, and they don't see eachother often. Well, this summer the boy was diagnosed with leukemia.. And well my other half always asks me if he's going to be okay, or if he's going to live.. And I never know what to tell him, or how to act. I don't want to lie to him, but I also don't want to hurt him or get his hopes up, because I don't really know the severity of the cancer.. I love this guy, and his little brother.. What do you say to someone who asks you such a question?