How to explain death to a toddler?

My dad had a heart attack yesterday right before thanksgiving dinner. He was playing with my 2 year old son when it happened. After calling 911 and handing it over to another family member I took Logan (my son) inside and he was saying “papa fall” over and over again and trying to go back out to him. He had a very close bond with his papa. I’ve been up all night consoling my mom and crying our eyes out. We are all still in shock. My son is still sleeping and I know when he gets up he will be asking for his papa. I don’t know what to do. I know he won’t understand. What would you say to him?