I cannot even express at how sorry I am to hear this. My brother-in-law was recently diagnosed with a stage 4 Glioblastoma brain tumor, but he was lucky enough to have it removed however still could have less then 2 yrs to live. But he is an adult and I don't deal with death well but it will be easier because he is an adult. No little precious angel should have to go through this at such a young age. I have not talked to God in 3 yrs since he took my mother-in-law away from us without warning but for this, I will be praying for you and your family. I dont really know how you will find the strength to tell your son or what you will say but I know when you need to do it, you will know. "You never know how strong you are until being strong is the only choice you have."
Your XI family will be here for you, and if you need to talk you now where to find me and all of us.
Also, I wanted to say on a good note congrats on the birth of Xander, he is adorable:)