How do I explain to my 5 year old that his Grandfather is very sick and may not make it?

My sons' grandfather is very sick with pneumonia and is is critical care hooked up to a ventilator, there is a chance he may not make it through. We have been to the hospital but he has not been able to see him. I want to explain to him the severity of his illness without scaring him. Please help with some advice on this tough issue!