What Do I Do/tell My Boyfriend When His Grandmother Who Is Like A Mother To Him Is Dying?

This is tough and you may not need to tell him anything, or try to say much as his loss may be hard to comprehend, but you can be there for him to lean on to talk to, to cry with, to do the practical things he will not want to do at the moment like cooking a meal, doing the laundry and cleaning the house. Just generally keeping his world going, will be the best thing you can do.  When someone we love is dying it is devastating, and we search for the right things to say to people. Yet, perhaps, what they want to hear is, I will be here for you, I don't understand why this is happening now, but when you  need me I will listen, and  let you talk about your loss, without silencing you or trying to avoid the subject.
That is all he needs, you as his support and time. Time is the great healer.