Thank you everyone.

I've just had a shot at explaining it to DS. He seems ok at the moment and has been asking questions. Thankfully his GGF wasn't in hospital so I don't have to try and explain about hospitals not always meaning death.
I'm not totally sure if we are going with the heaven thing as we don't really have a faith so I went with a star in the sky.
I felt I needed to explain why our family will all be upset for a few days.
He was really good about it, and I've let the rest of the family know how I've explained things so that they can expect lots of questions or DS talking about him.
DS is now off watching a movie, but randomly comes up and gives me cuddles and tells me he loves me.

Thanks again! I guess we see how it all goes at the funeral.