It's going to be hard for everyone, but since your son has special days with his great grandfather, he's going to ask questions if you try to beat around the bush.
Since your grandfather isn't doing so great and may not wake up, taking your son to see him should come with a little warning. While your son knows that he is sick, he doesn't know that he isn't going to get any better and that there aren't going to be anymore Tuesday outings. Not looking like he usually does may frighten your son, but it might get him to understand how grave his condition is as well.
It's horrible to think about, but your son will understand better than you may think. Just explain that he is very sick and the only way he can be better is to go on to someplace else. (Insert religion or philosophy here.) Yes, there are going to be questions about it, but you know what answers you have for yourself, so tell those to your son.
Personally, I remember asking about death when I was only a few years old. It scared me. I remember walking around after thinking about it for a few days and randomly bursting into tears and explaining to my grandmother that I didn't want to die. (My great uncle had recently passed and caused all of these things to come up.) She simply explained to me that it was okay and she didn't want to die either. She assured me that I wasn't going to die anytime soon and that she would go long before me. (She passed away almost two years ago now.)
That experience has stayed with me. My grandmother was honest and told me what she thought and treated me not like a child but like an equal, so I did understand that I would never be able to see those loved ones that had passed away anymore. Yes, it's hard, but at that same time, it almost makes it easier to take when you're told ahead of time. It is much harder to take a death when it just happens and isn't something that is just a matter of time.
If you trust that your son will understand, tell him whatever you believe he will be able to comprehend at this point. As time goes on, he will remember and ask more questions. Do not push him however, as my mother has issues because she had to go to a funeral as a child and didn't really understand death at that point. To this day, it seems to frighten and bother her because of that. It's really dependent upon the individual, so I think you should ask your son if he really wants to say goodbye or not.