Olivia, a small correction and a comment.
The Bible does not state Jonah was swallowed by a whale. Scripture states,
Now the Lord had prepared a great fish to swallow up Jonah
. Jonah 1:17a. It might have been a whale, it might not. We don't know (and I personally doubt it; not the swallowing, the whale).
Secondly, and this is far more serious as far as your pastor and his grandson go. The Master verified the story of Jonah.
But Jesus replied, “Only an evil, faithless nation would ask for further proof; and none will be given except what happened to Jonah the prophet! For as Jonah was in the great fish for three days and three nights, so I, the Messiah,[a] shall be in the heart of the earth three days and three nights
Matthew 12: 39-40
I am amazed your pastor does not know what Jesus said of Jonah. I assume he does and has told his grandson a little white lie to sustain his faith. In either case he is wrong and, though this may be difficult to do, you ought to bring it to his attention.