J.B.
I am so sorry you are going through so much right now. Wow, I think the best bet is just to tell them that soon grandma will go to live with God in Heaven and one day you will all be together again. It sounds like you have a sensitive son with a tender heart. It may hurt him, but that is such pure love that he has, I think I would just comfort him and remind him that God is taking really good care of grandma and grandpa and they are really happy with him, they aren't sick etc. If he continues to ask questions about it, just pray and answer whatever the Lord gives you to say, He knows how to comfort better than anyone. Take care of yourself, I will pray for you guys!