You should open your Bible and read the entire story yourself and with your grandson. Jonah refused to go to the Ninivites (enemies of the Isrealites) and preach to them. When he tries to run away by sailing off in the opposite direction, God causes a storm at sea and Jonah is then swallowed by the whale (or big fish). Jonah is spit out 3 days later on the shores of Ninivah.
He does preach and the people repent, so God spares the city. This angers Jonah, who then sulks and God needs to teach him another lesson.
The story has quite a bit of application in our own lives as we busily go about avoiding God’s will and thinking we know better than He does. It was well known to Jesus who uses it as an allegory when He talks about His resurrection.
Marine biologists have speculated on what type of whale/fish it might have been and there is actually quite a bit written about that. Maybe an interesting thing to research.
However, whether or not the story is literally true, it is still a story that teaches us quite a bit. And the faith that God can indeed do anything, is something that it would be good if your grandson developed. However, if he is a young child and just being cocky, I wouldn’t worry about it too much.
PS-If he is a young child, the veggietales version is hilarious.