It would depend upon the full situation and state negligence laws as to how an insurance company would "rule" on this incident. If anything this would fall under a homeowners policy or personal liabiliy umbrella.

If your son was taking due care in mowing the lawn and it was just an unfortunate accident then insurance may or may not cover it. Just as if the person put this claim through their own comprehensive coverage their insurance company may or may not come back and subrogate with you for the cost of repairs. You would need to know about your state's negligence laws and how they apply to this situation. Your insurance agent may be able to guide your or you could contact a lawyer to get this type of information.

If you or your son have already admitted responsibility and said you would pay for it then if you do not and are taken to court by the owner of the damaged van then you may need to explain why you no longer want to pay.

I have seen on court shows in the past were a person was mowing a lawn and the judge did find them liable for the damage that a rock caused to a passing car. In this particular instance though the person in the car won the case because they took pictures of the lawn showing that there were many rocks and that the person mowing had not taken reasonable care to avoid that patch of rocks that indeed did get spit up by the lawn mower.

So depending upon your state's negligence laws it may depend on the circumstances and the condition of the lawn. If you or your son had knowledge that there was a rocky part of the lawn and did not take care to not mow this section or to take out the rocks before mowing it then could be an issue.

You mentioned that you considered this the same as a car on the roadway kicking up a rock. It may be and again you should get counsel and information on your state's negligence laws to determine if you can be held liable. We will mention though in some situations if a rock is kicked up in the roadway that the DOT can be held responsible.

For example, section 768.28 of Florida Statutes, makes DOT liable as individuals would be liable for certain incidents. If the DOT is negligent or knew of a dangerous condition and did not correct it in a reasonable amount of time, DOT must pay. So if there is a pothole (and debris) that harm a car the key will be whether the DOT had notice of its existence and whether the DOT failed to fix it in a reasonable amount of time. If they were aware and are found negligence to have not fixed it they can be held liable for car damages resulting from debris coming loose and damaging cars.