David,
It is with an equal heavy heart that I read of your grandson. To see young ones snatched from a full life can be overwhelming.
May God infuse him with an indominatable spirit that he turn this into something heroic and show the docs he's down, but not out!
Attitude is critical. If he can fight and it's God's will, he may yet prove them wrong.
If not, prayers for his inner strength, through the Holy Spirit make him like a young chap I knew years ago who drove a motorized wheel chair with his chin and did data entry with a stick in his teeth.
He was a fighter and did well!
Praying for God's mercy to release your grandson from this prognosis and if not, to make him a champion among men in other ways!
Keep the Son in your eyes!