What to say to him when his grandfather dies?

My boyfriends grandfather is probably going to die in the next couple of days. I don't really know what to say to him when it happens. His grandfather influenced him and his family a lot. He had a stroke and he's been suffering in In the nursing home in California for a while. I'm pretty sure he's on hospice right now. So any advice what to say when it actually happens? I don't want to seem insensitive but I don't want to say the wrong thing.