It is very common for a child this age to regress during a stressful situation. He is also probably seeing your stress and others over his grandpa. That is a lot for a kid to understand and process. You might be thinking you are keeping it from him, but kids pick up on changes in their parents so easily. I think talking to him, maybe getting that book, would be good. He might be scared of you dying or himself, who knows, kids get some pretty wild ideas when they can't understand the situation. Good luck.