My son drove my car into a ditch. The tow truck operator winched it out,
but slightly damaged a panel. He pointed it out and said it was
unavoidable. It isn't much, but any sheet metal damage is expensive to
fix.

The car was a 2001 Golf in perfect condition.
There was no damage from going in the ditch.
The damage was a slightly bent panel behind the driver's front wheel, and
damage to the paint there. My insurance would go up way too much to file a
claim on it.
There is a responsible witness to the operators pointing out the damage he
had done.
Legally, the liability is clear; that is not the issue.

Several people have implied that I am a lousy father for letting my son
avoid his responsibilty; but that is largely the issue. If the tow truck
operator is responsible, then my son is not (and visa-versa). I want to
hold the right party responsibilty. Eventhough I could recover from the tow
truck company, I don't want to do it if it is wrong.