Let's be real, he thought that as soon as he heard his son was in a car crash. He just waited til he knew he was okay before telling the joke to anyone.
A colleague of mine has been known as "wump" for years. I didn't think anything of it until someone asked why that was his nickname, and one of the old boys said:
"that was the sound he made when he got run over about twenty years ago"
Similar story, I had a coworker who went by Skippy and it turns out people call him that because he still walks with a limp from a car crash that killed his brother.
He thought of the potential for that joke fifteen years before the crash and has been waiting patiently to drop that bomb. Just popped into his head in the shower one day and he buried the acorn in his brain and now we all have a beautiful tree to look at.