The part I'm having trouble with is that there was no damage from going into the ditch, but there was damage in removing it from the ditch. I can see two likely scenarios:
1 - Car goes into ditch big time, sustains damage and is in the ditch in such a way that hooking on to it for removal with a conventional tow would cause some slight additional damage.2- Car just barely goes into ditch, with no damage, in which case I would think it should be able to be removed without damage.
As Toller pointed out, the tow operator should have told his son upfront that he could not remove it without damaging it, in which case he could have pursued other options. If he simply removed it and damaged it in the process, then I would pursue a claim against the tow company. Worst case, you could take them to small claims court. Since you have a witness, you should be in a pretty good position.