"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. "

How much is the damage from removing the car as compared to the damage
it already had from going into the ditch? I would think there would be
damage from it going into the ditch to begin with. If the additional
damage is only a slightly damaged panel and there was already other
damage, then it may not cost that much additional to repair. If a body
shop has to repair and spray an an adjacent area, same color, etc, then
fixing the additonal damage shouldn't cost that much more.

I don;t see how after the fact you are going to be able to determine if
the additional damage could have been avoided. Depending on state laws
and who called the tow truck, you may not have much of a case, if at
all. For example, if the police called them, you may find that they
have the right to remove a car from the road right of way by any
reasonable means. If the damage is only a couple hundred bucks, I
would just have Jr pay for it.

You don't mention if you have collision insurance, sounds like you
don't, otherwise it shouldn't be an issue. BTW, are you sure you're
getting the true story from Jr? He's not trying to weasel out of the
damage by claiming it was not from the accident, is he?